/* ============================================================
   python-cert — Neon Blue & Silver Theme
   Dark = original cyber-night.  Light = brushed silver + neon.
   ============================================================ */

:root {
    --font-ui:    'Inter', system-ui, -apple-system, 'Segoe UI', Roboto, sans-serif;
    --font-mono:  'JetBrains Mono', 'Fira Code', 'Cascadia Code', Menlo, Consolas, monospace;
}

:root, [data-theme="dark"] {
    /* Background layers */
    --bg-base:           #050b16;
    --bg-deep:           #07111e;
    --bg-primary:        #050b16;
    --bg-secondary:      #08131f;
    --bg-card:           rgba(14, 24, 38, 0.72);
    --bg-elevated:       rgba(20, 32, 48, 0.8);
    --code-bg:           #060c14;

    --metal-1:           #2a3848;
    --metal-2:           #3a4a5e;
    --metal-3:           #5d6f80;
    --metal-light-1:     #b9c5d2;
    --metal-light-2:     #dbe5ed;
    --metal-edge:        rgba(220, 232, 245, 0.18);

    --blue-neon:         #39c2ff;
    --blue-glow:         rgba(57, 194, 255, 0.55);
    --blue-soft:         rgba(57, 194, 255, 0.18);
    --blue-deep:         #1666a0;
    --blue-text:         #9ddfff;

    --python-yellow:     #ffd43b;

    --text-primary:      #d8e6f0;
    --text-secondary:    #8da4b9;
    --text-muted:        #5d748a;

    --border:            rgba(57, 194, 255, 0.16);
    --border-strong:     rgba(57, 194, 255, 0.35);

    --success:           #39ff9c;
    --success-glow:      rgba(57, 255, 156, 0.40);
    --danger:            #ff4d6d;
    --danger-glow:       rgba(255, 77, 109, 0.40);
    --warning:           #ffd56b;

    --accent:            #39c2ff;
    --accent-hover:      #9ddfff;

    --shadow-glow:       0 0 32px rgba(57, 194, 255, 0.18);
    --shadow-deep:       0 12px 48px rgba(0, 0, 0, 0.55);

    /* Theme-specific composites */
    --backdrop:
        radial-gradient(ellipse 70% 50% at 30% 0%, rgba(57, 194, 255, 0.10), transparent 60%),
        radial-gradient(ellipse 50% 40% at 80% 100%, rgba(255, 212, 59, 0.04), transparent 65%),
        linear-gradient(180deg, #061427 0%, #050b16 60%, #050b16 100%);
    --header-bg:         linear-gradient(180deg, rgba(20, 32, 48, 0.7), rgba(8, 14, 24, 0.85));
    --card-bg:           linear-gradient(160deg, rgba(20, 32, 48, 0.85), rgba(8, 14, 24, 0.95));
    --hover-shadow:      0 12px 32px rgba(0, 0, 0, 0.5);
    --result-success-bg: rgba(57, 255, 156, 0.07);
    --result-failure-bg: rgba(255, 77, 109, 0.07);
    --title-gradient:    linear-gradient(135deg, #ffffff 0%, var(--blue-text) 50%, var(--blue-neon) 100%);
    --btn-shadow:        inset 0 1px 0 rgba(255,255,255,0.06), 0 2px 6px rgba(0,0,0,0.4);
    --btn-primary-shadow:
        inset 0 1px 0 rgba(255,255,255,0.4),
        0 0 18px var(--blue-glow),
        0 6px 18px rgba(0,0,0,0.4);

    --sidebar-bg:        rgba(8, 14, 24, 0.78);
    --sidebar-header-bg: rgba(20, 32, 48, 0.6);
    --hover-bg:          rgba(57, 194, 255, 0.06);
    --hover-bg-strong:   rgba(57, 194, 255, 0.12);
    --accent-bg:         rgba(57, 194, 255, 0.12);
}

[data-theme="light"] {
    --bg-base:           #dde4ec;
    --bg-deep:           #c8d2dd;
    --bg-primary:        #dde4ec;
    --bg-secondary:      #e6ecf2;
    --bg-card:           rgba(255, 255, 255, 0.78);
    --bg-elevated:       rgba(255, 255, 255, 0.92);
    --code-bg:           #1a2230;

    --metal-1:           #b9c4d0;
    --metal-2:           #ccd6e1;
    --metal-3:           #dee6ee;
    --metal-light-1:     #6a7a8c;
    --metal-light-2:     #5b6b7d;
    --metal-edge:        rgba(40, 60, 80, 0.20);

    --blue-neon:         #0e7cc2;
    --blue-glow:         rgba(14, 124, 194, 0.45);
    --blue-soft:         rgba(14, 124, 194, 0.14);
    --blue-deep:         #0d5b8f;
    --blue-text:         #1166a0;

    --python-yellow:     #b88a00;

    --text-primary:      #1c2530;
    --text-secondary:    #4a5868;
    --text-muted:        #748192;

    --border:            rgba(14, 124, 194, 0.25);
    --border-strong:     rgba(14, 124, 194, 0.50);

    --success:           #1a7d40;
    --success-glow:      rgba(26, 125, 64, 0.30);
    --danger:            #b32639;
    --danger-glow:       rgba(179, 38, 57, 0.30);
    --warning:           #b58410;

    --accent:            #0e7cc2;
    --accent-hover:      #1166a0;

    --shadow-glow:       0 0 24px rgba(14, 124, 194, 0.18);
    --shadow-deep:       0 8px 28px rgba(60, 80, 100, 0.18);

    --backdrop:
        radial-gradient(ellipse 70% 50% at 30% 0%, rgba(14, 124, 194, 0.10), transparent 60%),
        radial-gradient(ellipse 50% 40% at 80% 100%, rgba(184, 138, 0, 0.05), transparent 65%),
        linear-gradient(180deg, #e8eef4 0%, #dde4ec 60%, #d4dce6 100%);
    --header-bg:         linear-gradient(180deg, rgba(255, 255, 255, 0.85), rgba(225, 234, 244, 0.90));
    --card-bg:           linear-gradient(160deg, rgba(255, 255, 255, 0.95), rgba(232, 240, 248, 0.92));
    --hover-shadow:      0 12px 28px rgba(60, 80, 100, 0.20);
    --result-success-bg: rgba(26, 125, 64, 0.08);
    --result-failure-bg: rgba(179, 38, 57, 0.08);
    --title-gradient:    linear-gradient(135deg, var(--text-primary) 0%, var(--blue-deep) 50%, var(--blue-neon) 100%);
    --btn-shadow:        inset 0 1px 0 rgba(255,255,255,0.7), 0 2px 6px rgba(60,80,100,0.18);
    --btn-primary-shadow:
        inset 0 1px 0 rgba(255,255,255,0.5),
        0 0 14px var(--blue-glow),
        0 4px 12px rgba(60, 80, 100, 0.20);

    --sidebar-bg:        rgba(255, 255, 255, 0.82);
    --sidebar-header-bg: rgba(232, 240, 248, 0.65);
    --hover-bg:          rgba(14, 124, 194, 0.07);
    --hover-bg-strong:   rgba(14, 124, 194, 0.14);
    --accent-bg:         rgba(14, 124, 194, 0.14);
}

* { box-sizing: border-box; margin: 0; padding: 0; }

html, body {
    min-height: 100vh;
    background: var(--bg-base);
    color: var(--text-primary);
    font-family: var(--font-ui);
    font-size: 14px;
    line-height: 1.55;
    -webkit-font-smoothing: antialiased;
    overflow-x: hidden;
    transition: background-color 0.3s ease, color 0.3s ease;
}

/* Animated grid + radial backdrop */
body::before {
    content: '';
    position: fixed;
    inset: 0;
    z-index: -2;
    background: var(--backdrop);
    transition: background 0.3s ease;
}
body::after {
    content: '';
    position: fixed;
    inset: 0;
    z-index: -1;
    background-image:
        linear-gradient(var(--border) 1px, transparent 1px),
        linear-gradient(90deg, var(--border) 1px, transparent 1px);
    background-size: 48px 48px;
    mask-image: radial-gradient(ellipse 80% 60% at 50% 30%, black 30%, transparent 80%);
    -webkit-mask-image: radial-gradient(ellipse 80% 60% at 50% 30%, black 30%, transparent 80%);
    animation: gridDrift 60s linear infinite;
    opacity: 0.6;
}
@keyframes gridDrift {
    from { background-position: 0 0, 0 0; }
    to   { background-position: 48px 48px, 48px 48px; }
}
@keyframes pulse {
    0%, 100% { opacity: 1; }
    50%      { opacity: 0.55; }
}

.container {
    max-width: 1100px;
    margin: 0 auto;
    padding: 20px;
}

/* Header */
.header {
    background: var(--header-bg);
    border-bottom: 1px solid var(--border);
    padding: 22px 0;
    margin-bottom: 30px;
    backdrop-filter: blur(16px);
    -webkit-backdrop-filter: blur(16px);
    position: sticky;
    top: 0;
    z-index: 100;
    transition: background 0.3s ease, border-color 0.3s ease;
}

.header .container {
    display: flex;
    align-items: center;
    justify-content: space-between;
    gap: 20px;
}

.header-titles {
    display: flex;
    flex-direction: column;
    gap: 4px;
    min-width: 0;
}

.header-row {
    display: flex;
    justify-content: space-between;
    align-items: center;
    gap: 16px;
}

.header-actions {
    display: flex;
    align-items: center;
    gap: 10px;
    flex-shrink: 0;
}

.header h1 {
    font-size: 1.75rem;
    font-weight: 700;
    letter-spacing: -0.01em;
    display: flex;
    align-items: center;
    gap: 14px;
    background: var(--title-gradient);
    -webkit-background-clip: text;
    background-clip: text;
    color: transparent;
}

.header h1::before {
    content: '';
    width: 36px; height: 36px;
    border-radius: 10px;
    background: conic-gradient(from 220deg, var(--blue-neon), var(--blue-deep), var(--metal-2), var(--blue-neon));
    box-shadow:
        0 0 22px var(--blue-glow),
        inset 0 0 0 1px var(--metal-edge);
    flex-shrink: 0;
    -webkit-background-clip: border-box;
    background-clip: border-box;
}

.header .subtitle {
    color: var(--text-secondary);
    font-size: 0.9rem;
    margin-left: 50px;
    display: block;
}

/* Theme Toggle */
.theme-toggle {
    position: relative;
    width: 56px;
    height: 28px;
    background: var(--bg-deep);
    border: 1px solid var(--border-strong);
    border-radius: 14px;
    cursor: pointer;
    padding: 0;
    flex-shrink: 0;
    transition: all 0.2s ease;
}
.theme-toggle:hover {
    border-color: var(--blue-neon);
    box-shadow: 0 0 14px var(--blue-soft);
}
.theme-toggle::before, .theme-toggle::after {
    position: absolute;
    top: 50%;
    transform: translateY(-50%);
    font-size: 11px;
    color: var(--text-muted);
    pointer-events: none;
    line-height: 1;
}
.theme-toggle::before {
    content: "\263D";
    left: 8px;
}
.theme-toggle::after {
    content: "\2600";
    right: 8px;
}
.theme-toggle-thumb {
    position: absolute;
    top: 2px;
    left: 2px;
    width: 22px;
    height: 22px;
    background: var(--blue-neon);
    border-radius: 50%;
    transition: transform 0.25s cubic-bezier(0.4, 0, 0.2, 1), background-color 0.2s;
    box-shadow: 0 0 12px var(--blue-glow);
}
[data-theme="light"] .theme-toggle-thumb {
    transform: translateX(28px);
}

/* Cards */
.card {
    background: var(--card-bg);
    border: 1px solid var(--border);
    border-radius: 16px;
    padding: 22px 24px;
    margin-bottom: 14px;
    transition: transform .25s ease, border-color .25s ease, box-shadow .25s ease, background 0.3s ease;
    position: relative;
    overflow: hidden;
    backdrop-filter: blur(14px);
    -webkit-backdrop-filter: blur(14px);
}
.card::before {
    content: '';
    position: absolute;
    inset: 0;
    background: radial-gradient(circle at 80% 0%, var(--blue-glow), transparent 50%);
    opacity: 0.10;
    pointer-events: none;
    transition: opacity .25s ease;
}
.card::after {
    content: '';
    position: absolute;
    left: 0; right: 0; bottom: 0;
    height: 2px;
    background: linear-gradient(90deg, transparent, var(--blue-neon), transparent);
    opacity: 0.4;
    transition: opacity .25s ease;
}
.card:hover {
    transform: translateY(-2px);
    border-color: var(--border-strong);
    box-shadow: var(--hover-shadow), 0 0 24px var(--blue-soft);
}
.card:hover::before { opacity: 0.22; }
.card:hover::after  { opacity: 1; }

.card h2 {
    font-size: 1.15rem;
    margin-bottom: 8px;
    color: var(--text-primary);
    font-weight: 600;
    display: flex;
    align-items: center;
    gap: 8px;
    flex-wrap: wrap;
}

.card .meta {
    color: var(--text-muted);
    font-size: 0.85rem;
    margin-bottom: 12px;
    display: flex;
    align-items: center;
    gap: 8px;
    flex-wrap: wrap;
}

.card p { color: var(--text-secondary); }

/* Badges */
.badge {
    display: inline-block;
    padding: 2px 10px;
    border-radius: 999px;
    font-family: var(--font-mono);
    font-size: 0.7rem;
    font-weight: 600;
    text-transform: uppercase;
    letter-spacing: 0.08em;
    border: 1px solid currentColor;
    background: var(--blue-soft);
}
.badge-beginner     { color: var(--blue-neon); }
.badge-intermediate { color: var(--python-yellow); }
.badge-advanced     { color: #ff8b6b; }
[data-theme="light"] .badge-advanced { color: #c45a3a; }
.badge-completed {
    color: var(--success);
    border-color: var(--success);
    background: var(--result-success-bg);
    text-shadow: 0 0 4px var(--success-glow);
}

/* Category */
.category-header {
    display: flex;
    align-items: center;
    gap: 12px;
    font-family: var(--font-mono);
    font-size: 0.8rem;
    color: var(--blue-text);
    margin: 28px 0 14px;
    padding: 0;
    border: none;
    text-transform: uppercase;
    letter-spacing: 0.18em;
    font-weight: 500;
}
.category-header::before {
    content: '';
    width: 8px; height: 8px;
    border-radius: 50%;
    background: var(--blue-neon);
    box-shadow: 0 0 10px var(--blue-glow);
    animation: pulse 2.4s ease-in-out infinite;
}
.category-header::after {
    content: '';
    flex: 1;
    height: 1px;
    background: linear-gradient(90deg, var(--border-strong), transparent);
}

/* Code editor (raw textarea fallback) */
.code-editor {
    width: 100%;
    min-height: 300px;
    background: var(--code-bg);
    color: #d8e6f0;
    border: 1px solid var(--border);
    border-radius: 10px;
    padding: 16px;
    font-family: var(--font-mono);
    font-size: 14px;
    line-height: 1.5;
    tab-size: 4;
    resize: vertical;
}
.code-editor:focus {
    outline: none;
    border-color: var(--blue-neon);
    box-shadow: 0 0 0 1px var(--blue-neon), 0 0 16px var(--blue-soft);
}

/* CodeMirror polish — editor stays dark in both themes for code legibility */
.CodeMirror {
    border-radius: 10px !important;
    box-shadow: 0 0 0 1px var(--border), 0 8px 24px rgba(0, 0, 0, 0.45);
    transition: box-shadow .2s ease;
}
.CodeMirror-focused {
    box-shadow: 0 0 0 1px var(--blue-neon), 0 0 22px var(--blue-soft) !important;
}

/* Buttons */
.btn {
    display: inline-flex;
    align-items: center;
    gap: 8px;
    padding: 9px 20px;
    border-radius: 8px;
    font-family: var(--font-ui);
    font-size: 0.9rem;
    font-weight: 500;
    letter-spacing: 0.02em;
    cursor: pointer;
    text-decoration: none;
    border: 1px solid var(--border);
    background: linear-gradient(180deg, var(--metal-2), var(--metal-1));
    color: var(--text-primary);
    box-shadow: var(--btn-shadow);
    transition: all .15s ease;
}
.btn:hover {
    border-color: var(--border-strong);
    color: var(--blue-neon);
    box-shadow: var(--btn-shadow), 0 0 18px var(--blue-soft);
}
.btn:active { transform: translateY(1px); }

.btn-primary {
    background: linear-gradient(180deg, var(--blue-neon), var(--blue-deep));
    color: #ffffff;
    border-color: var(--blue-neon);
    text-shadow: 0 0 6px rgba(0,0,0,0.30);
    box-shadow: var(--btn-primary-shadow);
    font-weight: 600;
}
.btn-primary:hover {
    color: #ffffff;
    box-shadow: var(--btn-primary-shadow), 0 0 28px var(--blue-glow);
}
[data-theme="dark"] .btn-primary { color: #04101c; text-shadow: 0 0 6px rgba(255,255,255,0.25); }

.btn-secondary {
    background: linear-gradient(180deg, var(--metal-2), var(--metal-1));
    color: var(--text-secondary);
}
.btn-secondary:hover { color: var(--blue-neon); }

.btn-group {
    display: flex;
    gap: 10px;
    margin-top: 20px;
    flex-wrap: wrap;
}

/* Result */
.result-box {
    border-radius: 12px;
    padding: 18px;
    margin-top: 20px;
    backdrop-filter: blur(12px);
    -webkit-backdrop-filter: blur(12px);
    border: 1px solid var(--border);
    position: relative;
    overflow: hidden;
}
.result-box::before {
    content: '';
    position: absolute;
    inset: 0 0 auto 0;
    height: 1px;
    background: linear-gradient(90deg, transparent, currentColor, transparent);
    opacity: 0.7;
}
.result-success {
    background: var(--result-success-bg);
    border-color: var(--success);
    color: var(--success);
    box-shadow: 0 0 20px var(--success-glow);
}
.result-failure {
    background: var(--result-failure-bg);
    border-color: var(--danger);
    color: var(--danger);
    box-shadow: 0 0 20px var(--danger-glow);
}
.result-box h3 { margin-bottom: 8px; font-weight: 600; }
.result-success h3 { color: var(--success); text-shadow: 0 0 8px var(--success-glow); }
.result-failure h3 { color: var(--danger); text-shadow: 0 0 8px var(--danger-glow); }
.result-box pre {
    font-family: var(--font-mono);
    font-size: 0.85rem;
    white-space: pre-wrap;
    color: var(--text-secondary);
    margin-top: 8px;
}

/* Concepts */
.concepts-section { margin-top: 32px; }
.concepts-section h2 {
    color: var(--blue-text);
    font-family: var(--font-mono);
    font-size: 0.85rem;
    letter-spacing: 0.18em;
    text-transform: uppercase;
    display: flex;
    align-items: center;
    gap: 12px;
    margin-bottom: 14px !important;
    font-weight: 500;
}
.concepts-section h2::before {
    content: '';
    width: 8px; height: 8px;
    border-radius: 50%;
    background: var(--blue-neon);
    box-shadow: 0 0 10px var(--blue-glow);
}
.concepts-section h2::after {
    content: '';
    flex: 1;
    height: 1px;
    background: linear-gradient(90deg, var(--border-strong), transparent);
}

.concept-card {
    background: var(--card-bg);
    border: 1px solid var(--border);
    border-radius: 12px;
    padding: 18px 22px 18px 26px;
    margin-bottom: 14px;
    backdrop-filter: blur(12px);
    -webkit-backdrop-filter: blur(12px);
    position: relative;
    overflow: hidden;
}
.concept-card::before {
    content: '';
    position: absolute;
    left: 0; top: 0; bottom: 0;
    width: 3px;
    background: linear-gradient(180deg, var(--blue-neon), transparent);
    box-shadow: 0 0 12px var(--blue-glow);
}
.concept-card h3 {
    color: var(--blue-neon);
    font-size: 1rem;
    margin-bottom: 10px;
    text-shadow: 0 0 6px var(--blue-soft);
    font-weight: 600;
}
.concept-card p, .concept-text {
    color: var(--text-secondary);
    font-size: 0.9rem;
    line-height: 1.65;
}
.concept-code {
    display: block;
    background: var(--code-bg);
    border: 1px solid var(--border);
    border-radius: 8px;
    padding: 12px 16px;
    margin: 10px 0;
    font-family: var(--font-mono);
    font-size: 0.85rem;
    line-height: 1.5;
    overflow-x: auto;
    color: #c8d4e0;
}
.concept-code code {
    font-family: inherit;
    background: none;
    padding: 0;
    border: none;
    color: inherit;
}
.concept-inline-code {
    background: var(--blue-soft);
    border: 1px solid var(--border);
    border-radius: 4px;
    padding: 1px 6px;
    font-family: var(--font-mono);
    font-size: 0.85em;
    color: var(--python-yellow);
}

/* Progress */
.progress-bar {
    display: flex;
    align-items: center;
    gap: 14px;
    margin-bottom: 24px;
    color: var(--text-secondary);
    font-size: 0.85rem;
    font-family: var(--font-mono);
    padding: 12px 18px;
    background: var(--bg-elevated);
    border: 1px solid var(--border);
    border-radius: 12px;
    backdrop-filter: blur(10px);
    -webkit-backdrop-filter: blur(10px);
}
.progress-fill {
    flex: 1;
    height: 6px;
    background: var(--blue-soft);
    border-radius: 3px;
    overflow: hidden;
}
.progress-fill-inner {
    height: 100%;
    background: linear-gradient(90deg, var(--blue-deep), var(--blue-neon));
    border-radius: 3px;
    transition: width 0.4s ease;
    box-shadow: 0 0 12px var(--blue-glow);
}

/* Challenge description */
.challenge-description {
    background: var(--bg-card);
    border: 1px solid var(--border);
    border-left: 3px solid var(--blue-neon);
    box-shadow: -10px 0 20px -10px var(--blue-soft);
    padding: 16px 20px;
    margin-bottom: 22px;
    border-radius: 0 12px 12px 0;
    backdrop-filter: blur(12px);
    -webkit-backdrop-filter: blur(12px);
    color: var(--text-primary);
}

/* Breadcrumb */
.breadcrumb {
    margin-bottom: 22px;
    font-size: 0.78rem;
    font-family: var(--font-mono);
    letter-spacing: 0.12em;
    text-transform: uppercase;
    color: var(--text-muted);
}
.breadcrumb-link {
    color: var(--blue-text);
    text-decoration: none;
    cursor: pointer;
    background: none;
    border: none;
    font-size: inherit;
    font-family: inherit;
    letter-spacing: inherit;
    text-transform: inherit;
    padding: 0;
    transition: color .15s ease, text-shadow .15s ease;
}
.breadcrumb-link:hover {
    color: var(--blue-neon);
    text-shadow: 0 0 6px var(--blue-glow);
}
.breadcrumb-sep { color: var(--text-muted); margin: 0 10px; opacity: 0.6; }
.breadcrumb-current { color: var(--text-primary); font-weight: 500; }
.breadcrumb-counter { color: var(--text-muted); margin-left: 10px; font-size: 0.75rem; }

a.challenge-link { color: var(--text-primary); text-decoration: none; display: block; }

/* ============================
   Sidebar / Challenge Tree
   ============================ */

.app-layout {
    display: flex;
    min-height: 100vh;
    align-items: stretch;
}
.app-main {
    flex: 1;
    min-width: 0;
}

.sidebar {
    width: 280px;
    flex-shrink: 0;
    background: var(--sidebar-bg);
    border-right: 1px solid var(--border);
    display: flex;
    flex-direction: column;
    transition: width 0.2s ease, background-color 0.2s;
    overflow: hidden;
    backdrop-filter: blur(14px);
    -webkit-backdrop-filter: blur(14px);
    position: sticky;
    top: 0;
    height: 100vh;
    z-index: 90;
}
.sidebar.collapsed { width: 44px; }

.sidebar-header {
    display: flex;
    align-items: center;
    gap: 8px;
    padding: 10px 12px;
    border-bottom: 1px solid var(--border);
    background: var(--sidebar-header-bg);
    min-height: 44px;
    backdrop-filter: blur(10px);
    -webkit-backdrop-filter: blur(10px);
}
.sidebar-toggle {
    background: transparent;
    border: none;
    color: var(--text-secondary);
    cursor: pointer;
    padding: 4px;
    border-radius: 6px;
    display: flex;
    align-items: center;
    justify-content: center;
    flex-shrink: 0;
    transition: all .15s ease;
}
.sidebar-toggle:hover {
    background: var(--hover-bg-strong);
    color: var(--blue-neon);
    box-shadow: 0 0 10px var(--blue-soft);
}
.sidebar-title {
    font-family: var(--font-mono);
    font-size: 0.7rem;
    font-weight: 600;
    letter-spacing: 0.18em;
    color: var(--blue-text);
    text-transform: uppercase;
    white-space: nowrap;
    opacity: 1;
    transition: opacity 0.15s;
}
.sidebar.collapsed .sidebar-title,
.sidebar.collapsed .sidebar-tree {
    opacity: 0;
    pointer-events: none;
}

.sidebar-tree {
    flex: 1;
    overflow-y: auto;
    padding: 10px 0;
    font-size: 0.85rem;
}

.tree-category { margin-bottom: 4px; }
.tree-category-header {
    display: flex;
    align-items: center;
    gap: 8px;
    padding: 6px 12px;
    cursor: pointer;
    color: var(--text-primary);
    user-select: none;
    white-space: nowrap;
    font-family: var(--font-mono);
    font-size: 0.74rem;
    letter-spacing: 0.1em;
    text-transform: uppercase;
    transition: background-color .12s;
}
.tree-category-header:hover { background: var(--hover-bg); }

.tree-chevron {
    display: inline-block;
    color: var(--text-muted);
    font-size: 0.7rem;
    width: 10px;
    transition: transform 0.2s ease;
    transform: rotate(-90deg);
}
.tree-category.expanded .tree-chevron {
    transform: rotate(0deg);
    color: var(--blue-neon);
}
.tree-category-name { font-size: 0.74rem; }

.tree-category-children { display: none; padding-left: 6px; }
.tree-category.expanded .tree-category-children { display: block; }

.tree-challenge {
    display: flex;
    align-items: center;
    gap: 8px;
    padding: 5px 12px 5px 28px;
    color: var(--text-secondary);
    text-decoration: none;
    cursor: pointer;
    border: none;
    background: none;
    font-family: inherit;
    font-size: 0.82rem;
    width: 100%;
    text-align: left;
    white-space: nowrap;
    overflow: hidden;
    text-overflow: ellipsis;
    transition: background-color .12s, color .12s, padding-left .12s;
}
.tree-challenge:hover {
    background: var(--hover-bg);
    color: var(--text-primary);
}
.tree-challenge-active {
    background: var(--accent-bg);
    color: var(--text-primary);
    border-left: 2px solid var(--blue-neon);
    padding-left: 26px;
    box-shadow: inset 6px 0 16px var(--blue-soft);
    text-shadow: 0 0 6px var(--blue-soft);
}
.tree-challenge-active:hover {
    background: var(--accent-bg);
    filter: brightness(1.12);
}

.tree-file-icon {
    color: var(--blue-neon);
    font-size: 0.78rem;
    flex-shrink: 0;
    opacity: 0.65;
}
.tree-challenge-active .tree-file-icon { opacity: 1; }
.tree-challenge-title {
    overflow: hidden;
    text-overflow: ellipsis;
    flex: 1;
}
.tree-check {
    color: var(--success);
    font-size: 0.85rem;
    flex-shrink: 0;
    text-shadow: 0 0 6px var(--success-glow);
}

@media (max-width: 768px) {
    .sidebar { width: 44px; }
    .sidebar:not(.collapsed) {
        position: fixed;
        z-index: 200;
        height: 100vh;
        width: 260px;
        box-shadow: 4px 0 14px rgba(0, 0, 0, 0.4);
    }
}

/* Scrollbar */
::-webkit-scrollbar { width: 10px; height: 10px; }
::-webkit-scrollbar-track { background: var(--bg-base); }
::-webkit-scrollbar-thumb {
    background: linear-gradient(180deg, var(--metal-2), var(--metal-1));
    border-radius: 8px;
    border: 2px solid var(--bg-base);
}
::-webkit-scrollbar-thumb:hover {
    background: linear-gradient(180deg, var(--blue-deep), var(--metal-2));
}

/* ============================================================
   GitHub auth — Sign-in button, chip, sync dot, disclosure modal
   ============================================================ */

.github-signin {
    display: inline-flex;
    align-items: center;
    gap: 7px;
    padding: 6px 12px;
    border-radius: 8px;
    background: linear-gradient(180deg, var(--metal-2), var(--metal-1));
    border: 1px solid var(--border);
    color: var(--text-primary);
    text-decoration: none;
    font-size: 0.85rem;
    font-weight: 500;
    box-shadow: inset 0 1px 0 rgba(255,255,255,0.06), 0 2px 6px rgba(0,0,0,0.30);
    transition: all .15s ease;
}
.github-signin:hover {
    border-color: var(--border-strong);
    color: var(--blue-neon);
    box-shadow: inset 0 1px 0 rgba(255,255,255,0.08), 0 0 16px var(--blue-soft);
}
.github-signin svg { flex-shrink: 0; }

.github-chip {
    display: inline-flex;
    align-items: center;
    gap: 8px;
    padding: 4px 6px 4px 4px;
    border-radius: 999px;
    background: linear-gradient(135deg, var(--metal-1), var(--metal-2) 60%, var(--metal-1));
    border: 1px solid var(--border);
    box-shadow: inset 0 1px 0 rgba(255,255,255,0.06), 0 2px 8px rgba(0,0,0,0.30);
}
.gh-avatar-wrap {
    position: relative;
    display: inline-flex;
    align-items: center;
    flex-shrink: 0;
}
.github-chip .gh-avatar {
    width: 24px; height: 24px;
    border-radius: 50%;
    border: 1px solid var(--blue-neon);
    box-shadow: 0 0 6px var(--blue-soft);
    display: block;
}
.gh-sync-dot {
    position: absolute;
    right: -2px; bottom: -2px;
    width: 10px; height: 10px;
    border-radius: 50%;
    border: 2px solid var(--bg-base);
    background: var(--text-secondary);
    transition: background-color .15s ease, box-shadow .15s ease;
    box-sizing: border-box;
}
.gh-sync-idle    { background: var(--text-secondary); }
.gh-sync-syncing { background: var(--warning); animation: ghSyncPulse 1.1s ease-in-out infinite; }
.gh-sync-success { background: var(--success); box-shadow: 0 0 6px var(--success-glow); }
.gh-sync-error   { background: var(--danger);  box-shadow: 0 0 6px var(--danger-glow); }
@keyframes ghSyncPulse {
    0%, 100% { transform: scale(1);   opacity: 1; }
    50%      { transform: scale(1.3); opacity: 0.7; }
}

.github-chip .gh-login {
    color: var(--text-primary);
    font-size: 0.82rem;
    font-weight: 500;
    max-width: 130px;
    overflow: hidden;
    text-overflow: ellipsis;
    white-space: nowrap;
}
.github-chip .gh-action {
    display: inline-flex;
    align-items: center;
    justify-content: center;
    width: 22px; height: 22px;
    border-radius: 50%;
    color: var(--text-secondary);
    text-decoration: none;
    font-size: 0.85rem;
    border: 1px solid transparent;
    transition: all .15s ease;
}
.github-chip .gh-action:hover {
    color: var(--blue-neon);
    background: var(--blue-soft);
    border-color: var(--border-strong);
}

/* Disclosure modal */
.signin-modal {
    position: fixed;
    inset: 0;
    z-index: 1000;
    display: flex;
    align-items: center;
    justify-content: center;
    background: rgba(0, 8, 20, 0.65);
    backdrop-filter: blur(8px);
    -webkit-backdrop-filter: blur(8px);
    padding: 20px;
}
.signin-modal[hidden] { display: none; }
.signin-modal-box {
    background: var(--card-bg);
    border: 1px solid var(--border-strong);
    border-radius: 16px;
    box-shadow: 0 20px 60px rgba(0, 0, 0, 0.6), 0 0 32px var(--blue-soft);
    padding: 26px 28px 22px;
    max-width: 520px;
    width: 100%;
    color: var(--text-primary);
    backdrop-filter: blur(14px);
}
.signin-modal-title {
    margin: 0 0 14px;
    font-size: 1.2rem;
    font-weight: 700;
    color: var(--blue-neon);
    text-shadow: 0 0 8px var(--blue-soft);
}
.signin-modal-body { font-size: 0.92rem; line-height: 1.55; color: var(--text-secondary); }
.signin-modal-body p { margin: 0 0 10px; }
.signin-modal-body code {
    font-family: var(--font-mono);
    font-size: 0.85em;
    background: var(--blue-soft);
    border: 1px solid var(--border);
    padding: 1px 6px;
    border-radius: 4px;
    color: var(--blue-text);
}
.signin-modal-actions {
    display: flex;
    gap: 10px;
    justify-content: flex-end;
    margin-top: 18px;
}