:root {
    --bg: #1e1e1e;
    --panel: #252526;
    --panel-2: #2d2d30;
    --border: #3c3c3c;
    --accent: #007acc;
    --accent-hover: #1c8fde;
    --text: #d4d4d4;
    --text-dim: #858585;
    --text-bright: #ffffff;
    --yellow: #dcdcaa;
    --green: #4ec9b0;
    --red: #f48771;
    --orange: #ce9178;
    --blue: #569cd6;
    --purple: #c586c0;
    --bg-translucent: rgba(30, 30, 30, 0.85);

    --font-ui: "Segoe UI", -apple-system, BlinkMacSystemFont, system-ui, sans-serif;
    --font-mono: "JetBrains Mono", "Fira Code", Consolas, "SF Mono", Menlo, monospace;

    --radius: 6px;
    --header-h: 56px;

    color-scheme: dark;
}

* {
    box-sizing: border-box;
}

html, body {
    margin: 0;
    padding: 0;
    background: var(--bg);
    color: var(--text);
    font-family: var(--font-ui);
    font-size: 15px;
    line-height: 1.5;
    -webkit-font-smoothing: antialiased;
}

a {
    color: var(--accent);
    text-decoration: none;
}

a:hover {
    color: var(--accent-hover);
    text-decoration: underline;
}

button {
    font-family: inherit;
    font-size: inherit;
}

:focus-visible {
    outline: 2px solid var(--accent);
    outline-offset: 2px;
    border-radius: var(--radius);
}

code, pre, .mono {
    font-family: var(--font-mono);
}

/* ============================================================
   Header
============================================================ */
.site-header {
    position: sticky;
    top: 0;
    z-index: 50;
    height: var(--header-h);
    display: flex;
    align-items: center;
    padding: 0 24px;
    background: var(--bg-translucent);
    backdrop-filter: blur(12px);
    -webkit-backdrop-filter: blur(12px);
    border-bottom: 1px solid var(--border);
}

.logo {
    font-family: var(--font-mono);
    font-size: 18px;
    font-weight: 600;
    color: var(--text-bright);
    margin-right: 32px;
    display: inline-flex;
    align-items: center;
    gap: 8px;
}

.logo-accent {
    color: var(--accent);
}

.logo-icon {
    width: 28px;
    height: 28px;
    border-radius: 7px;
    object-fit: cover;
    display: block;
    flex-shrink: 0;
}

.logo:hover .logo-icon {
    transform: scale(1.05) rotate(-3deg);
    transition: transform 0.2s cubic-bezier(0.2, 0.8, 0.2, 1);
}

.logo:hover {
    text-decoration: none;
}

.nav {
    display: flex;
    gap: 24px;
    flex: 1;
}

.nav a {
    color: var(--text);
    font-size: 14px;
}

.nav a.active {
    color: var(--text-bright);
}

.nav a:hover {
    color: var(--accent-hover);
    text-decoration: none;
}

.header-cta {
    margin-left: auto;
}

/* ============================================================
   Buttons
============================================================ */
.btn {
    display: inline-flex;
    align-items: center;
    justify-content: center;
    gap: 6px;
    padding: 8px 16px;
    border: 1px solid var(--border);
    border-radius: var(--radius);
    background: var(--panel-2);
    color: var(--text);
    font-size: 14px;
    cursor: pointer;
    text-decoration: none;
    transition:
        background 0.18s ease,
        border-color 0.18s ease,
        color 0.18s ease,
        transform 0.18s ease,
        box-shadow 0.18s ease;
    will-change: transform;
}

.btn:hover {
    background: var(--accent-hover);
    border-color: var(--accent-hover);
    color: var(--text-bright);
    text-decoration: none;
}

.btn:hover:not(:disabled) {
    transform: translateY(-1px);
    box-shadow: 0 6px 18px -8px rgba(0, 122, 204, 0.55);
}

.btn:active:not(:disabled) {
    transform: translateY(0);
    transition-duration: 0.05s;
}

.btn:disabled {
    opacity: 0.5;
    cursor: not-allowed;
}

.btn:disabled:hover {
    background: var(--panel-2);
    border-color: var(--border);
    color: var(--text);
}

.btn-primary {
    background: var(--accent);
    border-color: var(--accent);
    color: var(--text-bright);
}

.btn-primary:hover {
    background: var(--accent-hover);
    border-color: var(--accent-hover);
}

.btn-outline {
    background: transparent;
}

.btn-ghost {
    background: transparent;
    border-color: transparent;
}

.btn-ghost:hover {
    background: var(--panel-2);
    border-color: var(--border);
    color: var(--text-bright);
}

.btn-icon {
    width: 32px;
    height: 32px;
    padding: 0;
}

.btn svg {
    width: 16px;
    height: 16px;
    flex-shrink: 0;
}

/* ============================================================
   Layout helpers
============================================================ */
.container {
    max-width: 1200px;
    margin: 0 auto;
    padding: 0 24px;
}

.section {
    padding: 48px 0;
}

.section-title {
    font-size: 28px;
    font-weight: 600;
    color: var(--text-bright);
    margin: 0 0 24px 0;
}

/* ============================================================
   Landing / Hero
============================================================ */
.hero {
    padding: 64px 0;
    display: grid;
    grid-template-columns: 1fr 1fr;
    gap: 48px;
    align-items: center;
}

.hero h1 {
    font-size: 48px;
    font-weight: 700;
    color: var(--text-bright);
    margin: 0 0 16px 0;
    line-height: 1.15;
    letter-spacing: -0.02em;
}

.hero h1 .accent {
    background: linear-gradient(135deg, var(--accent) 0%, #4ec9b0 100%);
    -webkit-background-clip: text;
    -webkit-text-fill-color: transparent;
    background-clip: text;
}

.hero-sub {
    font-size: 18px;
    color: var(--text-dim);
    margin: 0 0 32px 0;
}

.hero-ctas {
    display: flex;
    gap: 12px;
    flex-wrap: wrap;
}

.mock-ide {
    background: var(--panel);
    border: 1px solid var(--border);
    border-radius: var(--radius);
    overflow: hidden;
    font-family: var(--font-mono);
    font-size: 13px;
    line-height: 1.6;
}

.mock-ide-titlebar {
    background: var(--panel-2);
    border-bottom: 1px solid var(--border);
    padding: 8px 12px;
    display: flex;
    align-items: center;
    gap: 6px;
}

.mock-ide-dot {
    width: 12px;
    height: 12px;
    border-radius: 50%;
    background: var(--border);
}

.mock-ide-dot.r { background: #ff5f56; }
.mock-ide-dot.y { background: #ffbd2e; }
.mock-ide-dot.g { background: #27c93f; }

.mock-ide-name {
    margin-left: 8px;
    color: var(--text-dim);
    font-size: 12px;
}

.mock-ide-body {
    padding: 16px;
    overflow-x: auto;
}

.mock-line {
    display: flex;
}

.mock-ln {
    color: var(--text-dim);
    width: 28px;
    text-align: right;
    padding-right: 16px;
    user-select: none;
}

.tok-keyword { color: var(--blue); }
.tok-builtin { color: var(--purple); }
.tok-string  { color: var(--orange); }
.tok-number  { color: #b5cea8; }
.tok-comment { color: #6a9955; font-style: italic; }
.tok-fn      { color: var(--yellow); }
.tok-op      { color: var(--text); }

/* ============================================================
   Stats
============================================================ */
.stats {
    display: grid;
    grid-template-columns: repeat(3, 1fr);
    gap: 16px;
}

.stat {
    background: var(--panel);
    border: 1px solid var(--border);
    border-radius: var(--radius);
    padding: 24px;
    text-align: center;
    transition: border-color 0.2s ease, transform 0.25s cubic-bezier(0.2, 0.8, 0.2, 1);
}

.stat:hover {
    border-color: var(--accent);
    transform: translateY(-2px);
}

.stat-value {
    font-size: 36px;
    font-weight: 700;
    color: var(--text-bright);
    font-family: var(--font-mono);
    line-height: 1;
}

.stat-label {
    margin-top: 8px;
    color: var(--text-dim);
    font-size: 14px;
}

/* ============================================================
   Categories grid
============================================================ */
.cat-grid {
    display: grid;
    grid-template-columns: repeat(3, 1fr);
    gap: 16px;
}

.cat-card {
    background: var(--panel);
    border: 1px solid var(--border);
    border-radius: var(--radius);
    padding: 20px;
    color: var(--text);
    display: flex;
    flex-direction: column;
    gap: 12px;
    text-decoration: none;
    transition:
        border-color 0.2s ease,
        background 0.2s ease,
        transform 0.25s cubic-bezier(0.2, 0.8, 0.2, 1),
        box-shadow 0.25s ease;
    will-change: transform;
}

.cat-card:hover {
    border-color: var(--accent);
    background: var(--panel-2);
    text-decoration: none;
    transform: translateY(-3px);
    box-shadow: 0 12px 30px -16px rgba(0, 122, 204, 0.5);
}

.cat-card:hover .cat-card-icon {
    transform: scale(1.08) rotate(-3deg);
}

.cat-card-icon {
    transition: transform 0.25s cubic-bezier(0.2, 0.8, 0.2, 1);
}

.cat-card-icon {
    width: 32px;
    height: 32px;
    color: var(--accent);
}

/* (transition declared above on .cat-card-icon) */

.cat-card-title {
    font-size: 16px;
    font-weight: 600;
    color: var(--text-bright);
}

.cat-card-meta {
    color: var(--text-dim);
    font-size: 12px;
}

.cat-card-desc {
    color: var(--text-dim);
    font-size: 13px;
    line-height: 1.5;
}

/* ============================================================
   Footer
============================================================ */
.site-footer {
    border-top: 1px solid var(--border);
    padding: 32px 0;
    color: var(--text-dim);
    font-size: 13px;
    margin-top: 64px;
}

.site-footer .footer-grid {
    display: flex;
    justify-content: space-between;
    flex-wrap: wrap;
    gap: 16px;
}

/* ============================================================
   Catalog page
============================================================ */
.filters {
    position: sticky;
    top: var(--header-h);
    z-index: 30;
    background: var(--bg-translucent);
    backdrop-filter: blur(12px);
    -webkit-backdrop-filter: blur(12px);
    border-bottom: 1px solid var(--border);
    padding: 16px 0;
}

.filters-row {
    display: flex;
    gap: 12px;
    align-items: center;
    flex-wrap: wrap;
}

.input, .select {
    background: var(--panel);
    border: 1px solid var(--border);
    border-radius: var(--radius);
    color: var(--text);
    padding: 8px 12px;
    font-family: inherit;
    font-size: 14px;
    min-width: 0;
}

.input:focus, .select:focus {
    border-color: var(--accent);
    outline: none;
}

.input.search {
    flex: 1;
    min-width: 200px;
}

.chips {
    display: flex;
    gap: 8px;
    flex-wrap: wrap;
}

.chip {
    background: var(--panel);
    border: 1px solid var(--border);
    border-radius: 999px;
    padding: 4px 12px;
    font-size: 13px;
    color: var(--text);
    cursor: pointer;
    transition:
        background 0.18s ease,
        border-color 0.18s ease,
        color 0.18s ease,
        transform 0.18s ease;
}

.chip:hover {
    border-color: var(--accent);
    color: var(--text-bright);
    transform: translateY(-1px);
}

.chip.active {
    background: var(--accent);
    border-color: var(--accent);
    color: var(--text-bright);
}

.task-list {
    margin-top: 24px;
    border: 1px solid var(--border);
    border-radius: var(--radius);
    overflow: hidden;
}

.task-row {
    display: grid;
    grid-template-columns: 60px 1fr 160px 120px 60px;
    gap: 16px;
    padding: 12px 16px;
    align-items: center;
    border-bottom: 1px solid var(--border);
    color: var(--text);
    transition: background 0.12s ease;
}

.task-row:last-child {
    border-bottom: none;
}

.task-row.head {
    background: var(--panel-2);
    color: var(--text-dim);
    font-size: 12px;
    text-transform: uppercase;
    letter-spacing: 0.04em;
    font-weight: 600;
}

.task-row.head:hover {
    background: var(--panel-2);
}

a.task-row {
    transition: background 0.18s ease, color 0.18s ease, padding-left 0.2s ease;
}

a.task-row:hover {
    background: var(--panel-2);
    text-decoration: none;
    color: var(--text-bright);
    padding-left: 22px;
}

.task-row .id {
    color: var(--text-dim);
    font-family: var(--font-mono);
    font-size: 13px;
}

.task-row .title {
    font-weight: 500;
}

.task-row .cat {
    color: var(--text-dim);
    font-size: 13px;
}

.task-row .status {
    text-align: center;
    color: var(--text-dim);
}

.task-row .status.solved {
    color: var(--green);
}

.badge {
    display: inline-flex;
    align-items: center;
    padding: 3px 10px;
    border-radius: 999px;
    font-size: 12px;
    font-weight: 500;
    border: 1px solid var(--border);
    background: var(--panel-2);
}

.badge.easy {
    color: var(--green);
    border-color: rgba(78, 201, 176, 0.4);
    background: rgba(78, 201, 176, 0.08);
}

.badge.medium {
    color: var(--yellow);
    border-color: rgba(220, 220, 170, 0.4);
    background: rgba(220, 220, 170, 0.08);
}

.badge.hard {
    color: var(--red);
    border-color: rgba(244, 135, 113, 0.4);
    background: rgba(244, 135, 113, 0.08);
}

.empty {
    padding: 48px 16px;
    text-align: center;
    color: var(--text-dim);
}

.load-more {
    display: flex;
    justify-content: center;
    margin: 24px 0;
}

.match-count {
    margin-left: auto;
    color: var(--text-dim);
    font-size: 13px;
}

/* ============================================================
   Task page
============================================================ */
body.task-page {
    height: 100vh;
    overflow: hidden;
    display: flex;
    flex-direction: column;
}

.task-shell {
    flex: 1;
    min-height: 0;
    display: grid;
    grid-template-columns: 1fr 6px 1fr;
    grid-template-rows: 1fr;
}

.task-pane {
    overflow: auto;
    background: var(--bg);
    min-width: 0;
    min-height: 0;
}

.task-pane.left {
    padding: 24px 32px;
    border-right: 1px solid var(--border);
}

.splitter {
    background: var(--border);
    cursor: col-resize;
    user-select: none;
    transition: background 0.12s ease;
}

.splitter:hover, .splitter.dragging {
    background: var(--accent);
}

.task-title {
    font-size: 26px;
    font-weight: 600;
    color: var(--text-bright);
    margin: 0 0 12px 0;
    line-height: 1.2;
}

.task-meta {
    display: flex;
    align-items: center;
    gap: 8px;
    margin-bottom: 24px;
    flex-wrap: wrap;
    font-size: 13px;
    color: var(--text-dim);
}

.task-meta .sep {
    color: var(--border);
}

.tag {
    background: var(--panel-2);
    border: 1px solid var(--border);
    border-radius: var(--radius);
    padding: 2px 8px;
    font-size: 12px;
    color: var(--text-dim);
}

.task-desc {
    color: var(--text);
    line-height: 1.7;
}

.task-desc h2, .task-desc h3 {
    color: var(--text-bright);
    margin-top: 32px;
    margin-bottom: 12px;
}

.task-desc code {
    background: var(--panel-2);
    border: 1px solid var(--border);
    border-radius: 4px;
    padding: 1px 6px;
    font-size: 0.9em;
    color: var(--orange);
}

.task-desc pre {
    background: var(--panel);
    border: 1px solid var(--border);
    border-radius: var(--radius);
    padding: 12px 16px;
    overflow-x: auto;
}

.task-desc pre code {
    background: none;
    border: none;
    padding: 0;
    color: var(--text);
    font-size: 13px;
}

.task-desc ul, .task-desc ol {
    padding-left: 24px;
}

.task-desc blockquote {
    margin: 0;
    border-left: 3px solid var(--accent);
    padding: 4px 12px;
    color: var(--text-dim);
}

.example {
    margin: 16px 0;
    border: 1px solid var(--border);
    border-radius: var(--radius);
    overflow: hidden;
}

.example-head {
    background: var(--panel-2);
    padding: 6px 12px;
    font-size: 12px;
    color: var(--text-dim);
    text-transform: uppercase;
    letter-spacing: 0.04em;
}

.example-grid {
    display: grid;
    grid-template-columns: 1fr 1fr;
    gap: 0;
}

.example-cell {
    padding: 12px 16px;
    background: var(--panel);
}

.example-cell + .example-cell {
    border-left: 1px solid var(--border);
}

.example-label {
    font-size: 11px;
    color: var(--text-dim);
    text-transform: uppercase;
    margin-bottom: 6px;
    letter-spacing: 0.04em;
}

.example-cell pre {
    margin: 0;
    background: transparent;
    border: none;
    padding: 0;
    font-family: var(--font-mono);
    font-size: 13px;
    color: var(--text-bright);
    white-space: pre-wrap;
    word-break: break-word;
}

.example-explanation {
    grid-column: 1 / -1;
    padding: 8px 16px;
    background: var(--bg);
    border-top: 1px solid var(--border);
    color: var(--text-dim);
    font-size: 13px;
}

.constraints {
    margin: 24px 0;
    padding: 12px 16px;
    background: var(--panel);
    border: 1px solid var(--border);
    border-radius: var(--radius);
    font-size: 14px;
}

.constraints-label {
    font-size: 11px;
    color: var(--text-dim);
    text-transform: uppercase;
    letter-spacing: 0.04em;
    margin-bottom: 6px;
}

/* ============================================================
   Editor pane
============================================================ */
.editor-pane {
    display: flex;
    flex-direction: column;
    background: var(--panel);
    overflow: hidden;
}

.tabs {
    display: flex;
    background: var(--bg);
    border-bottom: 1px solid var(--border);
    padding: 0 8px;
    flex-shrink: 0;
}

.tab {
    background: transparent;
    border: none;
    color: var(--text-dim);
    padding: 10px 16px;
    cursor: pointer;
    font-size: 13px;
    font-family: inherit;
    border-bottom: 2px solid transparent;
    margin-bottom: -1px;
}

.tab:hover {
    color: var(--text);
}

.tab {
    transition: color 0.18s ease, border-bottom-color 0.25s ease;
}

.tab[aria-selected="true"] {
    color: var(--text-bright);
    border-bottom-color: var(--accent);
}

.tabpanel {
    display: none;
    flex: 1;
    min-height: 0;
    flex-direction: column;
    overflow: hidden;
}

.tabpanel.active {
    display: flex;
}

#editor {
    flex: 1;
    min-height: 0;
    overflow: hidden;
    font-family: var(--font-mono);
    font-size: 14px;
    background: var(--bg);
}

#editor .cm-editor {
    height: 100%;
    background: var(--bg);
}

#editor .cm-scroller {
    font-family: var(--font-mono);
}

textarea.fallback-editor {
    flex: 1;
    width: 100%;
    background: var(--bg);
    color: var(--text);
    border: none;
    border-radius: 0;
    padding: 16px;
    font-family: var(--font-mono);
    font-size: 14px;
    resize: none;
    outline: none;
    line-height: 1.5;
    tab-size: 4;
}

.toolbar {
    display: flex;
    align-items: center;
    gap: 8px;
    padding: 8px 12px;
    background: var(--panel);
    border-top: 1px solid var(--border);
    border-bottom: 1px solid var(--border);
    flex-shrink: 0;
}

.toolbar .runtime-tag {
    margin-left: auto;
    font-family: var(--font-mono);
    font-size: 12px;
    color: var(--text-dim);
    padding: 4px 10px;
    background: var(--panel-2);
    border: 1px solid var(--border);
    border-radius: var(--radius);
}

.tabpanel.active {
    --output-h: 40%;
}

.output-resizer {
    height: 6px;
    background: var(--border);
    cursor: row-resize;
    flex-shrink: 0;
    transition: background 0.15s ease;
    position: relative;
}

.output-resizer::before {
    content: "";
    position: absolute;
    inset: -3px 0;
}

.output-resizer:hover,
.output-resizer.dragging,
.output-resizer:focus-visible {
    background: var(--accent);
    outline: none;
}

.output-pane {
    background: var(--panel);
    display: flex;
    flex-direction: column;
    height: var(--output-h);
    min-height: 80px;
    flex-shrink: 0;
    overflow: hidden;
}

.output-tabs {
    display: flex;
    background: var(--bg);
    border-bottom: 1px solid var(--border);
    padding: 0 8px;
    align-items: center;
    flex-shrink: 0;
}

.output-tabs .summary {
    margin-left: auto;
    padding-right: 8px;
    color: var(--text-dim);
    font-size: 12px;
    font-family: var(--font-mono);
}

.output-body {
    flex: 1;
    min-height: 0;
    overflow: auto;
    padding: 12px 16px;
    font-family: var(--font-mono);
    font-size: 13px;
    color: var(--text);
}

.output-body.empty-msg {
    color: var(--text-dim);
    font-style: italic;
}

.output-body pre {
    margin: 0;
    white-space: pre-wrap;
    word-break: break-word;
}

.output-body .err {
    color: var(--red);
}

.tests-summary {
    margin-bottom: 12px;
    font-size: 13px;
    font-family: var(--font-mono);
}

/* ============================================================
   Terminal
============================================================ */
.terminal {
    flex: 1;
    min-height: 0;
    display: flex;
    flex-direction: column;
    background: #181818;
    font-family: var(--font-mono);
    font-size: 13px;
    line-height: 1.55;
    color: var(--text);
    overflow: auto;
}

.term-output {
    flex: 1;
    padding: 12px 14px 6px;
    white-space: pre-wrap;
    word-break: break-word;
    color: var(--text);
}

.term-hint {
    color: var(--text-dim);
    font-style: italic;
}

.term-stderr {
    color: var(--red);
}

.term-stdin {
    color: var(--yellow);
}

.term-sys {
    color: var(--text-dim);
    font-style: italic;
}

.term-input-row {
    display: flex;
    align-items: center;
    gap: 10px;
    padding: 10px 14px 12px;
    background: #1f1f1f;
    border-top: 1px solid var(--accent);
    position: sticky;
    bottom: 0;
    box-shadow: 0 -10px 20px -16px rgba(0, 122, 204, 0.6);
}

.term-prompt {
    color: var(--accent);
    font-weight: 700;
    font-family: var(--font-mono);
    font-size: 16px;
    user-select: none;
    animation: termCaret 1.1s ease-in-out infinite;
}

@keyframes termCaret {
    50% { opacity: 0.35; }
}

#term-input {
    flex: 1;
    background: var(--bg);
    border: 1px solid var(--accent);
    border-radius: var(--radius);
    outline: none;
    color: var(--text-bright);
    font-family: var(--font-mono);
    font-size: 14px;
    padding: 8px 12px;
    transition: border-color 0.15s ease, box-shadow 0.15s ease;
    caret-color: var(--accent);
}

#term-input:focus {
    border-color: var(--accent-hover);
    box-shadow: 0 0 0 3px rgba(0, 122, 204, 0.18);
}

#term-input::placeholder {
    color: var(--text-dim);
    font-style: italic;
}

.test-case {
    border: 1px solid var(--border);
    border-radius: var(--radius);
    margin-bottom: 8px;
    overflow: hidden;
}

.test-case-head {
    display: flex;
    align-items: center;
    gap: 8px;
    padding: 8px 12px;
    background: var(--panel-2);
    font-size: 13px;
}

.test-case-head .name {
    flex: 1;
    color: var(--text);
}

.test-case-head .time {
    color: var(--text-dim);
    font-size: 12px;
}

.test-case-head .icon {
    width: 16px;
    height: 16px;
    flex-shrink: 0;
}

.test-case.ok .icon { color: var(--green); }
.test-case.fail .icon { color: var(--red); }

.test-case-body {
    padding: 12px;
    border-top: 1px solid var(--border);
    display: grid;
    grid-template-columns: 1fr 1fr 1fr;
    gap: 8px;
}

.test-case-body .col {
    background: var(--bg);
    border: 1px solid var(--border);
    border-radius: 4px;
    padding: 8px 10px;
    overflow: auto;
    max-height: 240px;
}

.test-case-body .col.expected {
    border-left: 3px solid var(--green);
}

.test-case-body .col.got {
    border-left: 3px solid var(--red);
}

.test-case-body .col-label {
    font-size: 11px;
    color: var(--text-dim);
    text-transform: uppercase;
    margin-bottom: 4px;
    letter-spacing: 0.04em;
}

.test-case-body pre {
    margin: 0;
    white-space: pre-wrap;
    word-break: break-word;
    font-size: 12px;
}

.solved-banner {
    background: rgba(78, 201, 176, 0.12);
    border: 1px solid rgba(78, 201, 176, 0.4);
    color: var(--green);
    padding: 8px 16px;
    margin-bottom: 16px;
    border-radius: var(--radius);
    font-size: 14px;
    display: inline-flex;
    align-items: center;
    gap: 8px;
}

/* ============================================================
   Animations
============================================================ */
@keyframes fadeUp {
    from {
        opacity: 0;
        transform: translateY(10px);
    }
    to {
        opacity: 1;
        transform: translateY(0);
    }
}

@keyframes fadeIn {
    from { opacity: 0; }
    to { opacity: 1; }
}

@keyframes pulse {
    0%, 100% { opacity: 1; }
    50% { opacity: 0.5; }
}

.hero h1,
.hero-sub,
.hero-ctas,
.mock-ide,
.stats,
.cat-grid,
.section-title,
.task-list,
.task-title,
.task-meta,
.task-desc,
.example,
.constraints {
    animation: fadeUp 0.5s cubic-bezier(0.2, 0.8, 0.2, 1) both;
}

.hero-sub { animation-delay: 0.05s; }
.hero-ctas { animation-delay: 0.1s; }
.mock-ide { animation-delay: 0.05s; }
.cat-grid { animation-delay: 0.1s; }
.task-meta { animation-delay: 0.04s; }
.task-desc { animation-delay: 0.08s; }
.example { animation-delay: 0.12s; }
.constraints { animation-delay: 0.16s; }

.task-row:not(.head) {
    animation: fadeUp 0.4s cubic-bezier(0.2, 0.8, 0.2, 1) both;
}

.test-case {
    animation: fadeUp 0.3s cubic-bezier(0.2, 0.8, 0.2, 1) both;
}

/* ============================================================
   Auth pages + user menu
============================================================ */
.auth-shell {
    min-height: calc(100vh - var(--header-h) - 80px);
    display: flex;
    align-items: center;
    justify-content: center;
    padding: 32px 16px;
}

.auth-card {
    background: var(--panel);
    border: 1px solid var(--border);
    border-radius: var(--radius);
    padding: 32px 36px;
    width: 100%;
    max-width: 420px;
    display: flex;
    flex-direction: column;
    gap: 18px;
    animation: fadeUp 0.4s cubic-bezier(0.2, 0.8, 0.2, 1);
}

.auth-card h1 {
    margin: 0;
    font-size: 26px;
    color: var(--text-bright);
    letter-spacing: -0.01em;
}

.auth-sub {
    margin: -8px 0 0 0;
    color: var(--text-dim);
    font-size: 14px;
}

.field {
    display: flex;
    flex-direction: column;
    gap: 6px;
}

.field-label {
    font-size: 12px;
    color: var(--text-dim);
    text-transform: uppercase;
    letter-spacing: 0.04em;
    font-weight: 600;
}

.field input {
    background: var(--bg);
    border: 1px solid var(--border);
    border-radius: var(--radius);
    color: var(--text-bright);
    padding: 10px 14px;
    font-family: inherit;
    font-size: 14px;
    outline: none;
    transition: border-color 0.15s ease, box-shadow 0.15s ease;
}

.field input:focus {
    border-color: var(--accent);
    box-shadow: 0 0 0 3px rgba(0, 122, 204, 0.18);
}

.field-hint {
    color: var(--text-dim);
    font-size: 12px;
}

.auth-error {
    background: rgba(244, 135, 113, 0.1);
    border: 1px solid rgba(244, 135, 113, 0.4);
    color: var(--red);
    padding: 10px 14px;
    border-radius: var(--radius);
    font-size: 13px;
}

.auth-submit {
    margin-top: 4px;
}

.auth-foot {
    text-align: center;
    color: var(--text-dim);
    font-size: 13px;
}

/* User menu in nav */
#user-menu {
    margin-left: auto;
    display: flex;
    align-items: center;
    gap: 8px;
}

.user-pill {
    display: flex;
    align-items: center;
    gap: 8px;
    padding: 4px 10px 4px 4px;
    background: var(--panel-2);
    border: 1px solid var(--border);
    border-radius: 999px;
    color: var(--text);
    cursor: pointer;
    position: relative;
    transition: border-color 0.15s ease, background 0.15s ease;
}

.user-pill:hover,
.user-pill:focus {
    border-color: var(--accent);
    outline: none;
}

.user-avatar {
    width: 24px;
    height: 24px;
    border-radius: 50%;
    background: var(--accent);
    color: var(--text-bright);
    display: inline-flex;
    align-items: center;
    justify-content: center;
    font-weight: 700;
    font-size: 12px;
    font-family: var(--font-mono);
}

.user-name {
    font-size: 13px;
    color: var(--text-bright);
    max-width: 120px;
    white-space: nowrap;
    overflow: hidden;
    text-overflow: ellipsis;
}

.user-dropdown {
    position: absolute;
    right: 0;
    top: calc(100% + 6px);
    background: var(--panel);
    border: 1px solid var(--border);
    border-radius: var(--radius);
    min-width: 200px;
    padding: 4px;
    opacity: 0;
    transform: translateY(-4px);
    pointer-events: none;
    transition: opacity 0.15s ease, transform 0.15s ease;
    z-index: 60;
    box-shadow: 0 8px 24px -10px rgba(0, 0, 0, 0.6);
}

.user-pill:hover .user-dropdown,
.user-pill:focus .user-dropdown,
.user-pill:focus-within .user-dropdown {
    opacity: 1;
    transform: translateY(0);
    pointer-events: auto;
}

.user-dropdown-head {
    padding: 10px 12px;
    border-bottom: 1px solid var(--border);
    margin-bottom: 4px;
}

.user-dropdown-name {
    color: var(--text-bright);
    font-size: 14px;
    font-weight: 600;
}

.user-dropdown-username {
    color: var(--text-dim);
    font-size: 12px;
    font-family: var(--font-mono);
}

.user-dropdown-item {
    display: block;
    width: 100%;
    text-align: left;
    background: transparent;
    border: none;
    color: var(--text);
    padding: 8px 12px;
    border-radius: 4px;
    cursor: pointer;
    font-size: 13px;
    font-family: inherit;
}

.user-dropdown-item:hover {
    background: var(--panel-2);
    color: var(--text-bright);
}

/* ============================================================
   Lessons catalog
============================================================ */
.lesson-grid {
    display: flex;
    flex-direction: column;
    gap: 32px;
}

.theme-block {
    display: flex;
    flex-direction: column;
    gap: 12px;
    animation: fadeUp 0.5s cubic-bezier(0.2, 0.8, 0.2, 1) both;
    animation-delay: calc(var(--theme-i, 0) * 0.05s);
}

.theme-head {
    display: grid;
    grid-template-columns: auto 1fr auto;
    grid-template-areas:
        "num info bar"
        "num info bar";
    gap: 16px;
    align-items: center;
    padding: 14px 18px;
    background: var(--panel-2);
    border: 1px solid var(--border);
    border-radius: var(--radius);
}

.theme-num {
    font-family: var(--font-mono);
    font-size: 20px;
    font-weight: 700;
    color: var(--accent);
    grid-area: num;
}

.theme-info {
    grid-area: info;
    min-width: 0;
}

.theme-name {
    font-size: 17px;
    font-weight: 600;
    color: var(--text-bright);
}

.theme-meta {
    display: flex;
    gap: 12px;
    color: var(--text-dim);
    font-size: 12px;
    margin-top: 2px;
    font-family: var(--font-mono);
}

.theme-bar {
    grid-area: bar;
    width: 120px;
    height: 4px;
    background: var(--bg);
    border-radius: 999px;
    overflow: hidden;
}

.theme-bar-fill {
    height: 100%;
    background: linear-gradient(90deg, var(--accent), var(--green));
    transition: width 0.3s ease;
}

.theme-lessons {
    display: grid;
    grid-template-columns: 1fr;
    gap: 8px;
    padding-left: 18px;
    border-left: 2px solid var(--border);
    margin-left: 14px;
}

.lesson-card {
    display: flex;
    align-items: center;
    gap: 20px;
    padding: 20px 24px;
    background: var(--panel);
    border: 1px solid var(--border);
    border-radius: var(--radius);
    color: var(--text);
    text-decoration: none;
    transition:
        border-color 0.2s ease,
        background 0.2s ease,
        transform 0.25s cubic-bezier(0.2, 0.8, 0.2, 1),
        box-shadow 0.25s ease;
}

.lesson-card:hover {
    border-color: var(--accent);
    background: var(--panel-2);
    text-decoration: none;
    transform: translateY(-2px);
    box-shadow: 0 14px 30px -16px rgba(0, 122, 204, 0.45);
}

.lesson-card:hover .lesson-card-arrow {
    transform: translateX(4px);
    color: var(--accent);
}

.lesson-card-num {
    font-family: var(--font-mono);
    font-size: 28px;
    font-weight: 700;
    color: var(--text-dim);
    min-width: 56px;
    text-align: center;
}

.lesson-card-body {
    flex: 1;
    display: flex;
    flex-direction: column;
    gap: 6px;
    min-width: 0;
}

.lesson-card-title {
    font-size: 17px;
    font-weight: 600;
    color: var(--text-bright);
}

.lesson-card-desc {
    color: var(--text-dim);
    font-size: 13px;
    line-height: 1.5;
}

.lesson-card-meta {
    display: flex;
    align-items: center;
    gap: 10px;
    flex-wrap: wrap;
    margin-top: 4px;
}

.lesson-card-min {
    color: var(--text-dim);
    font-size: 12px;
    font-family: var(--font-mono);
}

.lesson-status {
    margin-left: auto;
    display: inline-flex;
    align-items: center;
    gap: 4px;
    font-size: 12px;
    color: var(--text-dim);
    padding: 2px 10px;
    background: var(--panel-2);
    border: 1px solid var(--border);
    border-radius: 999px;
}

.lesson-status.done {
    color: var(--green);
    border-color: rgba(78, 201, 176, 0.4);
    background: rgba(78, 201, 176, 0.08);
}

.lesson-status.in-progress {
    color: var(--yellow);
    border-color: rgba(220, 220, 170, 0.4);
    background: rgba(220, 220, 170, 0.08);
}

.lesson-card-arrow {
    width: 20px;
    height: 20px;
    color: var(--text-dim);
    transition: transform 0.2s ease, color 0.2s ease;
    flex-shrink: 0;
}

/* ============================================================
   Lesson viewer
============================================================ */
.lesson-shell {
    max-width: 880px;
    margin: 0 auto;
    padding: 24px;
}

.lesson-back {
    display: inline-block;
    color: var(--text-dim);
    font-size: 13px;
    margin-bottom: 12px;
}

.lesson-back:hover {
    color: var(--accent-hover);
    text-decoration: none;
}

.lesson-header {
    margin-bottom: 24px;
}

.lesson-title {
    font-size: 28px;
    font-weight: 600;
    color: var(--text-bright);
    margin: 0 0 8px 0;
    letter-spacing: -0.01em;
}

.lesson-meta {
    display: flex;
    align-items: center;
    gap: 10px;
    margin-bottom: 14px;
    flex-wrap: wrap;
}

.progress {
    height: 4px;
    background: var(--panel-2);
    border-radius: 999px;
    overflow: hidden;
}

.progress-fill {
    height: 100%;
    background: linear-gradient(90deg, var(--accent), var(--green));
    transition: width 0.3s cubic-bezier(0.2, 0.8, 0.2, 1);
}

.progress-text {
    margin-top: 6px;
    font-size: 12px;
    color: var(--text-dim);
    font-family: var(--font-mono);
}

.slide-area {
    background: var(--panel);
    border: 1px solid var(--border);
    border-radius: var(--radius);
    padding: 32px 36px;
    min-height: 380px;
    animation: fadeUp 0.35s cubic-bezier(0.2, 0.8, 0.2, 1);
}

.slide {
    display: flex;
    flex-direction: column;
    gap: 16px;
}

.slide-tag {
    display: inline-block;
    align-self: flex-start;
    padding: 3px 10px;
    border-radius: 999px;
    font-size: 11px;
    font-weight: 600;
    letter-spacing: 0.05em;
    text-transform: uppercase;
    color: var(--text-dim);
    background: var(--panel-2);
    border: 1px solid var(--border);
}

.slide-tag.accent {
    color: var(--accent);
    border-color: rgba(0, 122, 204, 0.4);
    background: rgba(0, 122, 204, 0.08);
}

.slide-tag.warn {
    color: var(--yellow);
    border-color: rgba(220, 220, 170, 0.4);
    background: rgba(220, 220, 170, 0.08);
}

.slide-tag.green {
    color: var(--green);
    border-color: rgba(78, 201, 176, 0.4);
    background: rgba(78, 201, 176, 0.08);
}

.slide-h {
    font-size: 22px;
    color: var(--text-bright);
    margin: 0;
    line-height: 1.3;
}

.slide-body {
    color: var(--text);
    line-height: 1.7;
    font-size: 15px;
}

.slide-body h2, .slide-body h3 {
    color: var(--text-bright);
    margin-top: 20px;
    margin-bottom: 8px;
}

.slide-body code {
    background: var(--panel-2);
    border: 1px solid var(--border);
    border-radius: 4px;
    padding: 1px 6px;
    font-family: var(--font-mono);
    font-size: 13px;
    color: var(--orange);
}

.slide-body pre {
    background: var(--bg);
    border: 1px solid var(--border);
    border-radius: var(--radius);
    padding: 12px 16px;
    overflow-x: auto;
    font-family: var(--font-mono);
    font-size: 13px;
}

.slide-body pre code {
    background: none;
    border: none;
    padding: 0;
    color: var(--text);
}

.slide-body ul, .slide-body ol {
    padding-left: 22px;
}

.slide-body li {
    margin-bottom: 4px;
}

.slide-body blockquote {
    margin: 0;
    padding: 4px 14px;
    border-left: 3px solid var(--accent);
    color: var(--text-dim);
}

.slide-lead {
    color: var(--text);
    font-size: 15px;
    margin: 0;
}

/* Code card (used in code/interactive slides) */
.code-card {
    border: 1px solid var(--border);
    border-radius: var(--radius);
    background: var(--bg);
    overflow: hidden;
}

.code-card-head {
    display: flex;
    align-items: center;
    justify-content: space-between;
    padding: 8px 14px;
    background: var(--panel-2);
    border-bottom: 1px solid var(--border);
    font-size: 12px;
    color: var(--text-dim);
    font-family: var(--font-mono);
}

.code-block {
    margin: 0;
    padding: 14px 16px;
    font-family: var(--font-mono);
    font-size: 13px;
    color: var(--text);
    line-height: 1.55;
    overflow-x: auto;
    background: var(--bg);
    white-space: pre;
}

.code-card.editable .mini-editor {
    min-height: 160px;
}

.mini-editor .cm-editor {
    background: var(--bg);
}

.btn-sm {
    padding: 4px 10px;
    font-size: 12px;
}

.btn-sm svg {
    width: 12px;
    height: 12px;
}

.hint-box {
    background: rgba(220, 220, 170, 0.08);
    border: 1px solid rgba(220, 220, 170, 0.4);
    border-radius: var(--radius);
    padding: 10px 14px;
    color: var(--yellow);
    font-size: 14px;
    line-height: 1.5;
}

/* Mini terminal inside slide */
.mini-terminal {
    background: #181818;
    border: 1px solid var(--border);
    border-radius: var(--radius);
    overflow: hidden;
    display: flex;
    flex-direction: column;
    max-height: 280px;
}

.mini-term-output {
    flex: 1;
    overflow: auto;
    padding: 12px 14px;
    font-family: var(--font-mono);
    font-size: 13px;
    line-height: 1.55;
    white-space: pre-wrap;
    word-break: break-word;
    color: var(--text);
    min-height: 60px;
}

.mini-term-input {
    display: flex;
    align-items: center;
    gap: 10px;
    padding: 8px 14px 12px;
    background: #1f1f1f;
    border-top: 1px solid var(--accent);
}

.mini-term-input .term-prompt {
    color: var(--accent);
    font-weight: 700;
    font-family: var(--font-mono);
    font-size: 16px;
}

.mini-term-input input {
    flex: 1;
    background: var(--bg);
    border: 1px solid var(--accent);
    border-radius: var(--radius);
    outline: none;
    color: var(--text-bright);
    font-family: var(--font-mono);
    font-size: 13px;
    padding: 6px 10px;
}

.mini-term-input input:focus {
    border-color: var(--accent-hover);
    box-shadow: 0 0 0 3px rgba(0, 122, 204, 0.18);
}

/* Quiz */
.quiz-form {
    display: flex;
    flex-direction: column;
    gap: 8px;
    margin-top: 4px;
}

.quiz-option {
    display: flex;
    align-items: center;
    gap: 10px;
    padding: 12px 16px;
    background: var(--bg);
    border: 1px solid var(--border);
    border-radius: var(--radius);
    cursor: pointer;
    transition: border-color 0.15s ease, background 0.15s ease;
}

.quiz-option:hover {
    border-color: var(--accent);
    background: var(--panel-2);
}

.quiz-option input {
    accent-color: var(--accent);
    cursor: pointer;
}

.quiz-option-text {
    color: var(--text);
    font-size: 14px;
}

.quiz-option:has(input:checked) {
    border-color: var(--accent);
    background: rgba(0, 122, 204, 0.08);
}

.quiz-form .btn {
    align-self: flex-start;
    margin-top: 4px;
}

.quiz-feedback {
    padding: 14px 16px;
    border-radius: var(--radius);
    font-size: 14px;
    line-height: 1.5;
    margin-top: 4px;
    animation: fadeUp 0.3s ease both;
}

.quiz-feedback.correct {
    background: rgba(78, 201, 176, 0.1);
    border: 1px solid rgba(78, 201, 176, 0.4);
    color: var(--green);
}

.quiz-feedback.wrong {
    background: rgba(244, 135, 113, 0.1);
    border: 1px solid rgba(244, 135, 113, 0.4);
    color: var(--red);
}

/* Task link slide */
.task-cta {
    display: inline-flex;
    align-items: center;
    justify-content: space-between;
    gap: 16px;
    padding: 18px 24px;
    background: var(--bg);
    border: 1px solid var(--accent);
    border-radius: var(--radius);
    color: var(--text-bright);
    font-size: 16px;
    font-weight: 500;
    text-decoration: none;
    transition: background 0.15s ease, transform 0.2s ease;
}

.task-cta:hover {
    background: var(--accent);
    text-decoration: none;
    transform: translateY(-2px);
}

/* Slide nav */
.slide-nav {
    display: flex;
    align-items: center;
    justify-content: space-between;
    margin-top: 24px;
    gap: 16px;
}

.slide-dots {
    display: flex;
    gap: 6px;
    align-items: center;
}

.dot {
    width: 9px;
    height: 9px;
    border-radius: 50%;
    background: var(--panel-2);
    border: 1px solid var(--border);
    cursor: pointer;
    padding: 0;
    transition: background 0.15s ease, border-color 0.15s ease, transform 0.15s ease;
}

.dot:hover {
    border-color: var(--accent);
    transform: scale(1.2);
}

.dot.done {
    background: var(--accent);
    border-color: var(--accent);
}

.dot.active {
    background: var(--text-bright);
    border-color: var(--text-bright);
    transform: scale(1.3);
}

@media (max-width: 767px) {
    .lesson-shell {
        padding: 16px;
    }
    .slide-area {
        padding: 20px 18px;
    }
    .lesson-card {
        flex-direction: column;
        align-items: flex-start;
        padding: 16px;
    }
    .lesson-card-num {
        min-width: auto;
        font-size: 22px;
        text-align: left;
    }
    .lesson-card-arrow {
        display: none;
    }
}

/* ============================================================
   Responsive
============================================================ */
@media (max-width: 1199px) {
    .hero {
        grid-template-columns: 1fr;
        gap: 32px;
    }
    .cat-grid {
        grid-template-columns: repeat(2, 1fr);
    }
    .hero h1 {
        font-size: 40px;
    }
    .stats {
        grid-template-columns: repeat(3, 1fr);
    }
}

@media (max-width: 767px) {
    body, html {
        font-size: 14px;
    }
    .hero h1 {
        font-size: 32px;
    }
    .hero {
        padding: 32px 0;
    }
    .section {
        padding: 32px 0;
    }
    .container {
        padding: 0 16px;
    }
    .site-header {
        padding: 0 16px;
    }
    .nav {
        gap: 16px;
    }
    .nav a {
        font-size: 13px;
    }
    .stats {
        grid-template-columns: 1fr;
    }
    .cat-grid {
        grid-template-columns: 1fr;
    }
    .task-row {
        grid-template-columns: 40px 1fr 80px 40px;
    }
    .task-row .cat,
    .task-row.head .cat {
        display: none;
    }
    .task-shell {
        grid-template-columns: 1fr;
        grid-template-rows: 1fr 6px 1fr;
    }
    .splitter {
        cursor: row-resize;
    }
    .task-pane.left {
        border-right: none;
        border-bottom: 1px solid var(--border);
        padding: 16px;
    }
    .test-case-body {
        grid-template-columns: 1fr;
    }
    .example-grid {
        grid-template-columns: 1fr;
    }
    .example-cell + .example-cell {
        border-left: none;
        border-top: 1px solid var(--border);
    }
}

/* ============================================================
   Scrollbars (subtle)
============================================================ */
::-webkit-scrollbar {
    width: 12px;
    height: 12px;
}

::-webkit-scrollbar-track {
    background: var(--bg);
}

::-webkit-scrollbar-thumb {
    background: var(--panel-2);
    border-radius: 6px;
    border: 3px solid var(--bg);
}

::-webkit-scrollbar-thumb:hover {
    background: var(--border);
}