/* ============================================================================
 * Shared navigation — one implementation for all 13 pages.
 *
 * Reading model: the site is a linear course read mostly on a phone, so every
 * frequent action lives in the THUMB ZONE at the bottom of the screen, not in
 * a hamburger at the top. Layers:
 *
 *   .site-nav      sticky top bar  — identity, language, reading progress
 *   .reader-bar    fixed bottom bar (<=900px) — prev / contents / next
 *   .reader-sheet  bottom sheet    (<=900px) — sections + all lessons
 *   .toc-rail      sticky side rail (>=1200px) — sections, always visible
 *
 * Was duplicated in all 12 lesson files; edit here only.
 * ==========================================================================*/

.site-nav {
    position: sticky;
    top: 0;
    z-index: 500;
    margin: -2rem -1rem 2.5rem;
    background: var(--card-bg);
    border-bottom: 1px solid var(--border-color);
    box-shadow: 0 4px 24px rgba(90, 70, 45, .07);
}
.site-nav-inner {
    position: relative;
    max-width: 1180px;
    margin: 0 auto;
    display: flex;
    align-items: center;
    gap: 1rem;
    padding: 0.6rem 1.25rem;
}
.site-nav-brand {
    display: flex;
    align-items: center;
    gap: 0.6rem;
    text-decoration: none;
    flex-shrink: 0;
}
.site-nav-logo {
    width: 38px;
    height: 38px;
    border-radius: 10px;
    background: linear-gradient(135deg, var(--accent-cyan), var(--accent-purple));
    display: flex;
    align-items: center;
    justify-content: center;
    font-size: 1.15rem;
    box-shadow: 0 0 15px rgba(109, 69, 168, 0.4);
}
.site-nav-brand-name {
    font-weight: 700;
    letter-spacing: 0.3px;
    font-size: 1.05rem;
    color: var(--text-primary);
    white-space: nowrap;
}
.site-nav-toggle {
    display: none;
    margin-left: auto;
    background: rgba(var(--ink-rgb), 0.05);
    border: 1px solid var(--border-color);
    color: var(--text-primary);
    font-size: 1.3rem;
    line-height: 1;
    cursor: pointer;
    width: 44px;
    height: 44px;
    border-radius: 10px;
    align-items: center;
    justify-content: center;
}
.site-nav-menu {
    display: flex;
    align-items: center;
    gap: 0.35rem;
    margin-left: auto;
}
.site-nav-link {
    display: inline-flex;
    align-items: center;
    gap: 0.35rem;
    background: none;
    border: 1px solid transparent;
    color: var(--text-secondary);
    font-family: inherit;
    font-weight: 600;
    font-size: 0.92rem;
    padding: 0.5rem 0.85rem;
    border-radius: 10px;
    cursor: pointer;
    text-decoration: none;
    white-space: nowrap;
    transition: background 0.25s ease, color 0.25s ease, border-color 0.25s ease;
}
.site-nav-link:hover { color: var(--text-primary); background: rgba(var(--ink-rgb), 0.07); }
.site-nav-caret { font-size: 0.7em; opacity: 0.85; }
/* Syllabus dropdown */
.site-nav-dropdown { position: relative; }
.site-nav-dropdown-menu {
    position: absolute;
    top: calc(100% + 10px);
    left: 0;
    min-width: 320px;
    max-width: 88vw;
    list-style: none;
    margin: 0;
    padding: 0.4rem;
    background: var(--bg-color);
    border: 1px solid var(--border-color);
    border-radius: 14px;
    box-shadow: 0 18px 44px rgba(90, 70, 45, .07);
    opacity: 0;
    visibility: hidden;
    transform: translateY(-6px);
    transition: opacity 0.2s ease, transform 0.2s ease, visibility 0.2s;
    z-index: 60;
}
.site-nav-dropdown.open .site-nav-dropdown-menu { opacity: 1; visibility: visible; transform: translateY(0); }
.site-nav-dropdown-menu li { margin: 0; }
.site-nav-dropdown-menu a {
    display: flex;
    gap: 0.55rem;
    padding: 0.6rem 0.75rem;
    border-radius: 9px;
    text-decoration: none;
    color: var(--text-secondary);
    font-size: 0.9rem;
    font-weight: 500;
    line-height: 1.35;
    border: 1px solid transparent;
}
.site-nav-dropdown-menu a:hover { background: rgba(var(--ink-rgb), 0.06); color: var(--text-primary); }
.site-nav-dropdown-menu a.active {
    background: linear-gradient(135deg, rgba(14, 116, 144, 0.15), rgba(109, 69, 168, 0.15));
    color: var(--text-primary);
    border-color: rgba(14, 116, 144, 0.3);
}
.site-nav-dropdown-menu .num {
    color: var(--accent-cyan);
    font-weight: 700;
    font-variant-numeric: tabular-nums;
}
/* Prev / Next pager */
.site-nav-pager { display: flex; align-items: center; gap: 0.3rem; }
.site-nav-link.disabled { opacity: 0.35; pointer-events: none; cursor: default; }
/* Controls (language + theme) */
.site-nav-controls {
    display: flex;
    align-items: center;
    gap: 0.5rem;
    border-left: 1px solid var(--border-color);
    padding-left: 0.7rem;
    margin-left: 0.15rem;
}
.site-nav-icon-btn {
    background: rgba(var(--ink-rgb), 0.05);
    border: 1px solid var(--border-color);
    color: var(--text-primary);
    height: 40px;
    padding: 0 0.7rem;
    border-radius: 10px;
    cursor: pointer;
    font-weight: 600;
    font-size: 0.85rem;
    font-family: inherit;
    display: inline-flex;
    align-items: center;
    gap: 0.35rem;
    white-space: nowrap;
    transition: background 0.25s ease, border-color 0.25s ease;
}
.site-nav-icon-btn:hover { background: rgba(var(--ink-rgb), 0.1); border-color: var(--text-ghost); }

/* Reserve the scrollbar gutter so locking body scroll for the sheet does not
   reflow the whole page by a scrollbar's width. */
html { scrollbar-gutter: stable; }

/* --- Reading progress hairline (sits on the nav's bottom edge) ------------ */
.site-nav-progress {
    position: absolute;
    left: 0;
    bottom: -1px;
    height: 3px;
    width: 100%;
    transform: scaleX(0);
    transform-origin: 0 50%;
    background: linear-gradient(90deg, var(--accent-cyan), var(--accent-purple));
    pointer-events: none;
}

/* --- Lesson title in the top bar (mobile only) --------------------------- */
.site-nav-title {
    display: none;
    min-width: 0;
    flex: 1;
    margin: 0 0.5rem;
    font-family: var(--font-heading);
    font-weight: 600;
    font-size: 0.95rem;
    line-height: 1.3;
    color: var(--text-primary);
    overflow: hidden;
    text-overflow: ellipsis;
    white-space: nowrap;
}

/* --- Shared section list (used by both the sheet and the rail) ------------ */
.toc-list { list-style: none; margin: 0; padding: 0; }
.toc-list a {
    display: flex;
    gap: 0.6rem;
    align-items: baseline;
    padding: 0.75rem 0.7rem;
    border-radius: 9px;
    border-left: 2px solid transparent;
    text-decoration: none;
    color: var(--text-secondary);
    font-size: 0.9rem;
    line-height: 1.4;
    transition: background 0.2s ease, color 0.2s ease, border-color 0.2s ease;
}
.toc-list a:hover { background: rgba(var(--ink-rgb), 0.06); color: var(--text-primary); }
.toc-list a .num {
    color: var(--accent-cyan);
    font-weight: 700;
    font-variant-numeric: tabular-nums;
    flex-shrink: 0;
}
.toc-list a.current {
    background: var(--tint-cyan);
    border-left-color: var(--accent-cyan);
    color: var(--text-primary);
    font-weight: 600;
}

/* --- Desktop section rail (only where there is genuinely room) ------------ */
.toc-rail { display: none; }
@media (min-width: 1200px) {
    /* Shift the 860px reading column left so the rail has its own space
       instead of overlapping the prose. `html .container` outranks each
       lesson's own `.container { margin: 0 auto }`, declared later in the
       cascade, which would otherwise re-centre the column under the rail. */
    html .container {
        margin-left: max(1.5rem, calc(50% - 570px));
        margin-right: auto;
    }
    .toc-rail {
        display: block;
        position: fixed;
        top: 6.5rem;
        /* Same formula as the column's left edge, plus its 860px width and a
           32px gutter, so the rail tracks the column at every width. */
        left: calc(max(1.5rem, 50% - 570px) + 892px);
        width: 220px;
        max-height: calc(100vh - 9rem);
        overflow-y: auto;
        overscroll-behavior: contain;
        padding-left: 0.75rem;
        border-left: 1px solid var(--border-color);
        z-index: 40;
    }
    .toc-rail-heading {
        font-family: var(--font-heading);
        font-size: 0.72rem;
        font-weight: 700;
        letter-spacing: 0.08em;
        text-transform: uppercase;
        color: var(--text-muted);
        margin: 0 0 0.6rem 0.7rem;
    }
    .toc-rail .toc-list a { font-size: 0.84rem; padding: 0.4rem 0.6rem; }
}

/* --- Mobile: top bar slims down, everything else moves to the bottom ------ */
@media (max-width: 900px) {
    /* The pager, syllabus dropdown and "Hub" link all live in .reader-bar now,
       so the hamburger and its dropdown panel are gone entirely. */
    .site-nav-toggle,
    .site-nav-menu .site-nav-pager,
    .site-nav-menu .site-nav-dropdown,
    .site-nav-menu > a.site-nav-link { display: none; }
    .site-nav-title { display: block; }
    .site-nav-brand-name { display: none; }
    .site-nav-menu { margin-left: 0; }
    .site-nav-inner { gap: 0.5rem; padding: 0.5rem 0.85rem; }
    .site-nav-controls { border-left: none; padding-left: 0; margin-left: 0; }

    /* 44px minimum tap targets on touch screens. */
    .site-nav-brand { min-width: 44px; min-height: 44px; align-items: center; }
    .site-nav-icon-btn { min-width: 44px; min-height: 44px; }

    /* Room for the fixed bottom bar so the last card is never covered.
       `html body` outranks each lesson's own `padding` shorthand, which is
       declared later in the cascade and would otherwise reset this. */
    html body { padding-bottom: calc(76px + env(safe-area-inset-bottom, 0px)); }
}
@media (max-width: 768px) {
    .site-nav { margin: -1.25rem -0.85rem 2rem; }
}

/* --- The thumb zone: fixed bottom bar ------------------------------------ */
.reader-bar { display: none; }
@media (max-width: 900px) {
    .reader-bar {
        display: flex;
        position: fixed;
        left: 0;
        right: 0;
        bottom: 0;
        z-index: 600;
        align-items: stretch;
        gap: 0.35rem;
        padding: 0.4rem 0.5rem;
        padding-bottom: calc(0.4rem + env(safe-area-inset-bottom, 0px));
        background: var(--card-bg);
        border-top: 1px solid var(--border-color);
        box-shadow: 0 -6px 24px rgba(90, 70, 45, .09);
        transition: transform 0.25s ease;
    }
    /* Hidden while reading downward, back on the first upward scroll. */
    .reader-bar.tucked { transform: translateY(110%); }

    .reader-bar-step {
        display: flex;
        flex: 0 0 auto;
        align-items: center;
        justify-content: center;
        gap: 0.2rem;
        min-width: 64px;
        min-height: 48px;
        padding: 0 0.6rem;
        border: 1px solid var(--border-color);
        border-radius: 12px;
        background: var(--card-bg-alt);
        color: var(--text-secondary);
        font-family: inherit;
        font-weight: 600;
        font-size: 0.85rem;
        text-decoration: none;
        white-space: nowrap;
    }
    .reader-bar-step:active { background: rgba(var(--ink-rgb), 0.08); }
    .reader-bar-step.disabled { opacity: 0.35; pointer-events: none; }

    /* Centre control: current section readout AND the sheet trigger. */
    .reader-bar-toc {
        flex: 1 1 auto;
        min-width: 0;
        display: flex;
        align-items: center;
        gap: 0.55rem;
        min-height: 48px;
        padding: 0 0.75rem;
        border: 1px solid var(--accent-cyan);
        border-radius: 12px;
        background: var(--tint-cyan);
        color: var(--text-primary);
        font-family: inherit;
        cursor: pointer;
        text-align: left;
    }
    .reader-bar-toc-icon { flex-shrink: 0; font-size: 1rem; line-height: 1; }
    .reader-bar-toc-label {
        flex: 1;
        min-width: 0;
        overflow: hidden;
        text-overflow: ellipsis;
        white-space: nowrap;
        font-weight: 600;
        font-size: 0.88rem;
    }
    .reader-bar-toc-count {
        flex-shrink: 0;
        font-size: 0.78rem;
        font-weight: 700;
        font-variant-numeric: tabular-nums;
        color: var(--accent-cyan);
    }
}

/* --- The contents sheet -------------------------------------------------- */
.reader-sheet-backdrop {
    position: fixed;
    inset: 0;
    z-index: 700;
    background: rgba(var(--ink-rgb), 0.42);
    opacity: 0;
    visibility: hidden;
    transition: opacity 0.25s ease, visibility 0s 0.25s;
}
.reader-sheet-backdrop.open {
    opacity: 1;
    visibility: visible;
    transition: opacity 0.25s ease, visibility 0s 0s;
}

.reader-sheet {
    position: fixed;
    left: 0;
    right: 0;
    bottom: 0;
    z-index: 710;
    display: flex;
    flex-direction: column;
    max-height: 76vh;
    background: var(--bg-color);
    border-top: 1px solid var(--border-color);
    border-radius: 20px 20px 0 0;
    box-shadow: 0 -18px 44px rgba(90, 70, 45, .16);
    transform: translateY(100%);
    visibility: hidden;
    /* visibility flips instantly on open (so the sheet can take focus) and
       waits for the slide-out to finish on close. */
    transition: transform 0.28s cubic-bezier(.32, .72, 0, 1), visibility 0s 0.28s;
    padding-bottom: env(safe-area-inset-bottom, 0px);
}
.reader-sheet.open {
    transform: translateY(0);
    visibility: visible;
    transition: transform 0.28s cubic-bezier(.32, .72, 0, 1), visibility 0s 0s;
}

.reader-sheet-grip {
    width: 40px;
    height: 4px;
    margin: 0.6rem auto 0.2rem;
    border-radius: 2px;
    background: var(--border-strong);
    flex-shrink: 0;
}
.reader-sheet-tabs {
    display: flex;
    gap: 0.35rem;
    padding: 0.6rem 1rem 0.75rem;
    border-bottom: 1px solid var(--border-color);
    flex-shrink: 0;
}
.reader-sheet-tab {
    flex: 1;
    min-height: 42px;
    border: 1px solid var(--border-color);
    border-radius: 10px;
    background: transparent;
    color: var(--text-secondary);
    font-family: inherit;
    font-weight: 600;
    font-size: 0.88rem;
    cursor: pointer;
}
.reader-sheet-tab[aria-selected="true"] {
    border-color: var(--accent-cyan);
    border-width: 2px;
    background: var(--tint-cyan);
    color: var(--text-primary);
}
.reader-sheet-body {
    overflow-y: auto;
    overscroll-behavior: contain;
    -webkit-overflow-scrolling: touch;
    padding: 0.7rem 1rem 1.1rem;
}
.reader-sheet-panel[hidden] { display: none; }

/* Lesson rows inside the sheet's "all lessons" panel. */
.sheet-lesson-list { list-style: none; margin: 0; padding: 0; }
.sheet-lesson-list a {
    display: flex;
    gap: 0.7rem;
    align-items: center;
    padding: 0.7rem;
    border-radius: 10px;
    border: 1px solid transparent;
    text-decoration: none;
    color: var(--text-secondary);
    font-size: 0.9rem;
    line-height: 1.4;
}
.sheet-lesson-list a.active {
    background: var(--tint-cyan);
    border-color: var(--accent-cyan);
    color: var(--text-primary);
    font-weight: 600;
}
.sheet-lesson-dot {
    flex-shrink: 0;
    width: 10px;
    height: 10px;
    border-radius: 50%;
    border: 2px solid var(--border-strong);
}
.sheet-lesson-dot.in-progress { border-color: var(--accent-orange); background: var(--accent-orange); }
.sheet-lesson-dot.completed  { border-color: var(--accent-green);  background: var(--accent-green); }
.sheet-lesson-list .num {
    color: var(--accent-cyan);
    font-weight: 700;
    font-variant-numeric: tabular-nums;
    flex-shrink: 0;
}
.sheet-links {
    display: flex;
    gap: 0.5rem;
    margin-top: 0.8rem;
    padding-top: 0.8rem;
    border-top: 1px solid var(--border-color);
}
.sheet-links a {
    flex: 1;
    min-height: 44px;
    display: flex;
    align-items: center;
    justify-content: center;
    border: 1px solid var(--border-color);
    border-radius: 10px;
    text-decoration: none;
    color: var(--text-secondary);
    font-weight: 600;
    font-size: 0.88rem;
}

/* --- Sideways-scrolling panels ------------------------------------------
   Wide diagrams and tables scroll inside their own container rather than
   breaking the page, but on a phone nothing says so. An inset shadow is
   painted against the padding box, so it stays pinned to the right edge while
   the content scrolls under it. Applied by site-nav.js only to panels that
   actually overflow, and removed once you reach the end. */
.x-scrollable {
    box-shadow: inset -18px 0 14px -14px rgba(var(--ink-rgb), 0.38);
    overscroll-behavior-x: contain;
    -webkit-overflow-scrolling: touch;
}
.x-scrollable.at-start { box-shadow: inset -18px 0 14px -14px rgba(var(--ink-rgb), 0.38); }
.x-scrollable.at-end   { box-shadow: inset  18px 0 14px -14px rgba(var(--ink-rgb), 0.38); }
.x-scrollable.at-both  {
    box-shadow: inset -18px 0 14px -14px rgba(var(--ink-rgb), 0.38),
                inset  18px 0 14px -14px rgba(var(--ink-rgb), 0.38);
}

/* --- End-of-lesson card: close the loop where the reading happens --------- */
.lesson-end {
    margin: 2rem 0 1rem;
    padding: 1.5rem;
    background: var(--card-bg);
    border: 1px solid var(--border-color);
    border-radius: 16px;
    box-shadow: var(--shadow-soft);
}
.lesson-end-title {
    font-family: var(--font-heading);
    font-size: 1.05rem;
    font-weight: 600;
    color: var(--text-primary);
    margin: 0 0 1rem;
}
.lesson-end-actions { display: flex; flex-wrap: wrap; gap: 0.7rem; }
.lesson-end-complete {
    display: inline-flex;
    align-items: center;
    gap: 0.45rem;
    min-height: 46px;
    padding: 0 1.1rem;
    border: 2px solid var(--accent-green);
    border-radius: 12px;
    background: var(--tint-green);
    color: var(--accent-green);
    font-family: inherit;
    font-weight: 700;
    font-size: 0.92rem;
    cursor: pointer;
}
.lesson-end-complete[aria-pressed="true"] {
    background: var(--accent-green);
    color: var(--text-on-accent);
}
.lesson-end-next {
    display: inline-flex;
    align-items: center;
    gap: 0.45rem;
    min-height: 46px;
    padding: 0 1.1rem;
    border: 1px solid var(--border-color);
    border-radius: 12px;
    background: var(--card-bg-alt);
    color: var(--text-primary);
    font-weight: 600;
    font-size: 0.92rem;
    text-decoration: none;
}
.lesson-end-next .label { color: var(--text-muted); font-weight: 500; }

/* --- Motion preferences -------------------------------------------------- */
@media (prefers-reduced-motion: reduce) {
    .reader-bar,
    .reader-sheet,
    .reader-sheet-backdrop { transition: none; }
    .reader-bar.tucked { transform: none; }
}