styles.css

/* ── Reset & base ──────────────────────────────────────── */
*, *::before, *::after { box-sizing: border-box; margin: 0; padding: 0; }
html { scroll-behavior: smooth; font-size: 16px; }

body {
  font-family: -apple-system, BlinkMacSystemFont, "SF Pro Text", "Segoe UI", Helvetica, Arial, sans-serif;
  background: #ffffff;
  color: #1d1d1f;
  line-height: 1.6;
  display: flex;
  flex-direction: column;
  min-height: 100vh;
}

/* ── Brand colors ──────────────────────────────────────── */
:root {
  --brand-navy:   #1F457F;
  --brand-amber:  #F2B13E;
  --brand-coral:  #E4565C;
  --brand-green:  #5FC56F;
  --brand-cyan:   #5AC6D2;
  --brand-white:  #FFFFFF;
  --topbar-h:     56px;
}

/* ── Top bar ───────────────────────────────────────────── */
.topbar {
  position: fixed;
  top: 0; left: 0; right: 0;
  height: var(--topbar-h);
  background: rgba(31, 69, 127, 0.96);
  backdrop-filter: saturate(180%) blur(20px);
  -webkit-backdrop-filter: saturate(180%) blur(20px);
  border-bottom: 1px solid rgba(255,255,255,0.1);
  display: flex;
  align-items: center;
  padding: 0 20px;
  gap: 10px;
  z-index: 1000;
}

.topbar-logo {
  font-size: 0.88rem;
  font-weight: 700;
  letter-spacing: -0.01em;
  color: var(--brand-white);
  text-decoration: none;
  white-space: nowrap;
  margin-right: 6px;
}
.topbar-logo span {
  display: inline-block;
  width: 28px; height: 28px;
  background: var(--brand-amber);
  border-radius: 7px;
  text-align: center;
  line-height: 28px;
  font-size: 0.9rem;
  margin-right: 8px;
  vertical-align: middle;
}

.topbar-divider {
  width: 1px; height: 22px;
  background: rgba(255,255,255,0.2);
  margin: 0 4px;
}

/* ── Nav pill dropdowns ─────────────────────────────────── */
.nav-pill {
  position: relative;
}
.nav-pill-btn {
  display: flex;
  align-items: center;
  gap: 6px;
  background: rgba(255,255,255,0.12);
  border: 1px solid rgba(255,255,255,0.2);
  border-radius: 20px;
  padding: 5px 14px 5px 12px;
  color: var(--brand-white);
  font-size: 0.8rem;
  font-weight: 500;
  cursor: pointer;
  white-space: nowrap;
  transition: background 0.15s;
  font-family: inherit;
}
.nav-pill-btn:hover {
  background: rgba(255,255,255,0.2);
}
.nav-pill-btn .pill-label {
  color: rgba(255,255,255,0.6);
  font-size: 0.72rem;
  margin-right: 2px;
}
.nav-pill-btn svg {
  opacity: 0.7;
  transition: transform 0.2s;
  flex-shrink: 0;
}
.nav-pill.open .nav-pill-btn svg {
  transform: rotate(180deg);
}

.nav-dropdown {
  display: none;
  position: absolute;
  top: calc(100% + 8px);
  left: 0;
  min-width: 260px;
  background: #1a2f52;
  border: 1px solid rgba(255,255,255,0.15);
  border-radius: 14px;
  padding: 8px 0;
  box-shadow: 0 20px 60px rgba(0,0,0,0.4);
  z-index: 2000;
}
.nav-pill.open .nav-dropdown { display: block; }

.nav-dropdown-heading {
  font-size: 0.62rem;
  font-weight: 700;
  letter-spacing: 0.1em;
  text-transform: uppercase;
  color: rgba(255,255,255,0.35);
  padding: 10px 16px 4px;
}
.nav-dropdown a {
  display: flex;
  align-items: center;
  gap: 10px;
  padding: 8px 16px;
  font-size: 0.82rem;
  color: rgba(255,255,255,0.75);
  text-decoration: none;
  transition: background 0.12s, color 0.12s;
}
.nav-dropdown a:hover {
  background: rgba(255,255,255,0.08);
  color: #fff;
}
.nav-dropdown a.current {
  color: var(--brand-amber);
  font-weight: 600;
}
.nav-dropdown a .dd-num {
  font-size: 0.7rem;
  color: rgba(255,255,255,0.3);
  min-width: 20px;
  font-variant-numeric: tabular-nums;
}
.nav-dropdown a.current .dd-num { color: var(--brand-amber); }
.nav-dropdown a .dd-check {
  margin-left: auto;
  color: var(--brand-green);
  font-size: 0.85rem;
}

.nav-dropdown-sep {
  height: 1px;
  background: rgba(255,255,255,0.1);
  margin: 6px 0;
}

/* ── Progress pill (right side) ─────────────────────────── */
.topbar-progress {
  margin-left: auto;
  display: flex;
  align-items: center;
  gap: 10px;
}
.progress-text {
  font-size: 0.75rem;
  color: rgba(255,255,255,0.55);
  white-space: nowrap;
}
.progress-bar-wrap {
  width: 80px;
  height: 4px;
  background: rgba(255,255,255,0.15);
  border-radius: 2px;
  overflow: hidden;
}
.progress-bar-fill {
  height: 100%;
  background: var(--brand-amber);
  border-radius: 2px;
  transition: width 0.4s ease;
  width: 0%;
}

/* ── Layout ────────────────────────────────────────────── */
.layout {
  display: flex;
  margin-top: var(--topbar-h);
  flex: 1;
}
/* ── Layout wrapper alias (.html uses page-wrap) ──────────── */
.page-wrap {
  display: flex;
  margin-top: var(--topbar-h);
  flex: 1;
}


/* ── Sidebar ───────────────────────────────────────────── */
.sidebar {
  width: 268px;
  min-width: 268px;
  background: #1F457F;
  color: #ebebf5;
  position: sticky;
  top: var(--topbar-h);
  height: calc(100vh - var(--topbar-h));
  overflow-y: auto;
  padding: 28px 0 40px;
  flex-shrink: 0;
  border-right: 1px solid rgba(255,255,255,0.1);
}
.sidebar::-webkit-scrollbar { width: 4px; }
.sidebar::-webkit-scrollbar-thumb { background: rgba(255,255,255,0.2); border-radius: 2px; }

.sidebar-chapter {
  font-size: 0.62rem;
  font-weight: 700;
  letter-spacing: 0.12em;
  text-transform: uppercase;
  color: rgba(255,255,255,0.45);
  padding: 0 22px;
  margin-bottom: 14px;
}
.sidebar nav a {
  display: flex;
  align-items: flex-start;
  gap: 10px;
  padding: 8px 22px;
  font-size: 0.82rem;
  font-weight: 400;
  color: rgba(255,255,255,0.5);
  text-decoration: none;
  border-left: 2px solid transparent;
  transition: color 0.15s, background 0.15s, border-color 0.15s;
  line-height: 1.4;
}
.sidebar nav a:hover {
  color: rgba(255,255,255,0.95);
  background: rgba(255,255,255,0.1);
}
.sidebar nav a.active {
  color: var(--brand-amber);
  border-left-color: var(--brand-amber);
  background: rgba(242,177,62,0.08);
  font-weight: 500;
}
.sidebar nav a .nav-num {
  min-width: 18px;
  font-size: 0.7rem;
  color: rgba(255,255,255,0.25);
  padding-top: 1px;
  font-variant-numeric: tabular-nums;
}
.sidebar nav a.active .nav-num { color: var(--brand-amber); opacity: 0.7; }

.sidebar nav a .nav-badge {
  font-size: 0.6rem;
  border-radius: 4px;
  padding: 1px 6px;
  margin-top: 2px;
  white-space: nowrap;
  font-weight: 600;
}
.badge-tutorial { background: rgba(90,198,210,0.15); color: var(--brand-cyan); }
.badge-exercise { background: rgba(242,177,62,0.15); color: var(--brand-amber); }

.sidebar nav a.active .nav-badge {
  background: rgba(242,177,62,0.2);
  color: var(--brand-amber);
}

/* Completed indicator in sidebar */
.sidebar nav a .nav-done {
  margin-left: auto;
  color: var(--brand-green);
  font-size: 0.85rem;
  opacity: 0;
  transition: opacity 0.2s;
}
.sidebar nav a.done .nav-done { opacity: 1; }
.sidebar nav a.done { color: rgba(255,255,255,0.35); }
.sidebar nav a.done.active { color: var(--brand-amber); }

/* ── Main content ──────────────────────────────────────── */
.content {
  flex: 1;
  min-width: 0;
  padding: 0 0 120px;
}

/* ── Section ───────────────────────────────────────────── */
section {
  scroll-margin-top: calc(var(--topbar-h) + 20px);
}
/* ── Section body content — indent to align with section-header */
.section > p,
.section > .code-wrap,
.section > .img-placeholder,
.section > .img-wrap,
.section > .im-table-wrap,
.section > .exercise-check,
.section > .solution-btn,
.section > .solution-body,
.section > h3,
.section > ul,
.section > ol {
  margin-left: 80px;
  margin-right: 80px;
}


/* ── Section header (03-06.html) ──────────────────────── */
.section-header {
  display: flex;
  align-items: flex-start;
  gap: 20px;
  padding: 40px 80px 24px;
  border-bottom: 1px solid #f0f0f5;
  max-width: 900px;
}
.section-num {
  font-size: 0.75rem;
  font-weight: 700;
  color: rgba(26,26,46,0.25);
  letter-spacing: 0.05em;
  min-width: 24px;
  padding-top: 6px;
}
.section-header h2 {
  font-size: 1.6rem;
  font-weight: 700;
  color: #1a1a2e;
  margin: 0 0 6px;
  line-height: 1.2;
}
.section-header .badge {
  font-size: 0.68rem;
  font-weight: 600;
  border-radius: 4px;
  padding: 2px 8px;
  letter-spacing: 0.04em;
}

/* ── Section hero ──────────────────────────────────────── */
.section-hero {
  padding: 64px 80px 48px;
  border-bottom: 1px solid #f0f0f5;
  max-width: 900px;
}

.section-tag {
  display: inline-flex;
  align-items: center;
  gap: 5px;
  font-size: 0.7rem;
  font-weight: 700;
  letter-spacing: 0.09em;
  text-transform: uppercase;
  border-radius: 6px;
  padding: 3px 9px;
  margin-bottom: 16px;
}
.section-tag.tutorial {
  background: rgba(90,198,210,0.12);
  color: #1a8a95;
}
.section-tag.exercise {
  background: rgba(242,177,62,0.12);
  color: #9a6800;
}

.section-hero h1 {
  font-size: 2.2rem;
  font-weight: 700;
  letter-spacing: -0.03em;
  line-height: 1.15;
  color: #1a1a2e;
  margin-bottom: 18px;
}
.section-hero .lead {
  font-size: 1.05rem;
  color: #4a5568;
  max-width: 620px;
  line-height: 1.7;
}

/* ── Complete button in hero ───────────────────────────── */
.complete-btn {
  display: inline-flex;
  align-items: center;
  gap: 8px;
  margin-top: 24px;
  padding: 9px 18px;
  border-radius: 22px;
  border: 1.5px solid #d1d5db;
  background: transparent;
  color: #6b7280;
  font-size: 0.82rem;
  font-weight: 500;
  cursor: pointer;
  font-family: inherit;
  transition: all 0.2s;
}
.complete-btn:hover {
  border-color: var(--brand-navy);
  color: var(--brand-navy);
  background: rgba(31,69,127,0.04);
}
.complete-btn.completed {
  border-color: var(--brand-green);
  color: #2e7d32;
  background: rgba(95,197,111,0.08);
}
.complete-btn .btn-icon { font-size: 1rem; }

/* ── Article body ──────────────────────────────────────── */
.article {
  max-width: 900px;
  padding: 0 80px 64px;
  border-bottom: 1px solid #f0f0f5;
}
.article:last-child { border-bottom: none; }

/* ── Image placeholder ─────────────────────────────────── */
.img-placeholder {
  background: #f8f9fb;
  border-radius: 16px;
  height: 320px;
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
  margin: 32px 0;
  border: 1.5px dashed #d1d5db;
  gap: 10px;
  color: #9ca3af;
}
.img-placeholder .img-icon { font-size: 2.2rem; }
.img-placeholder .img-name {
  font-size: 0.78rem;
  font-family: "SF Mono", "Fira Code", monospace;
  background: #e5e7eb;
  padding: 3px 10px;
  border-radius: 5px;
  color: #4b5563;
}
.img-placeholder .img-hint {
  font-size: 0.75rem;
  color: #b0b7c0;
}

/* Real images (replace placeholder once added) */
.img-wrap {
  margin: 32px 0;
  border-radius: 16px;
  overflow: hidden;
  border: 1px solid #e5e7eb;
}
.img-wrap img {
  width: 100%;
  height: auto;
  display: block;
}

/* ── Headings in article ───────────────────────────────── */
.article h2 {
  font-size: 1.5rem;
  font-weight: 700;
  letter-spacing: -0.02em;
  color: var(--brand-navy);
  margin: 40px 0 14px;
  padding-bottom: 8px;
  border-bottom: 2px solid rgba(31,69,127,0.1);
}
.article h3 {
  font-size: 1.05rem;
  font-weight: 600;
  color: #1a1a2e;
  margin: 28px 0 10px;
}
.article p {
  font-size: 0.96rem;
  color: #374151;
  margin-bottom: 14px;
  line-height: 1.7;
}
.article ul, .article ol {
  padding-left: 22px;
  margin-bottom: 14px;
}
.article li {
  font-size: 0.96rem;
  color: #374151;
  margin-bottom: 6px;
  line-height: 1.6;
}
.article strong { color: #1a1a2e; }

blockquote {
  border-left: 3px solid var(--brand-amber);
  padding: 10px 16px;
  margin: 20px 0;
  background: rgba(242,177,62,0.06);
  border-radius: 0 8px 8px 0;
  font-size: 0.93rem;
  color: #4a5568;
}

/* ── Code blocks ───────────────────────────────────────── */
.code-wrap {
  background: #163460;
  border-radius: 14px;
  margin: 20px 0;
  overflow: hidden;
}
.code-header {
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 10px 18px;
  background: #1F457F;
  border-bottom: 1px solid rgba(255,255,255,0.1);
}
.code-lang {
  font-size: 0.7rem;
  font-weight: 600;
  letter-spacing: 0.06em;
  text-transform: uppercase;
  color: rgba(255,255,255,0.35);
}
.code-copy {
  font-size: 0.72rem;
  color: rgba(255,255,255,0.35);
  cursor: pointer;
  background: none;
  border: none;
  font-family: inherit;
  padding: 2px 6px;
  border-radius: 4px;
  transition: color 0.15s, background 0.15s;
}
.code-copy:hover { color: #fff; background: rgba(255,255,255,0.1); }
pre {
  padding: 20px 22px;
  overflow-x: auto;
  font-family: "SF Mono", "Fira Code", "Fira Mono", "Roboto Mono", monospace;
  font-size: 0.85rem;
  line-height: 1.65;
  color: #cdd6f4;
  tab-size: 2;
}
/* Swift tokens */
.kw  { color: #cba6f7; }
.fn  { color: #89b4fa; }
.num { color: #fab387; }
.str { color: #a6e3a1; }
.cm  { color: #6c7086; font-style: italic; }
.vr  { color: #cdd6f4; }
.tp  { color: #f38ba8; }

/* ── Inline code ───────────────────────────────────────── */
code {
  font-family: "SF Mono", "Fira Code", monospace;
  font-size: 0.87em;
  background: #f1f5f9;
  border-radius: 5px;
  padding: 2px 6px;
  color: var(--brand-navy);
}

/* ── IM Table ──────────────────────────────────────────── */
.im-table-wrap {
  margin: 28px 0 8px;
  border-radius: 12px;
  overflow: hidden;
  border: 1px solid #e0e0e0;
}
.im-table-header {
  background: #c8c8c8;
  color: #4a4a4a;
  font-size: 0.72rem;
  font-weight: 700;
  letter-spacing: 0.08em;
  text-transform: uppercase;
  padding: 10px 16px;
}
table {
  width: 100%;
  border-collapse: collapse;
}
th {
  background: #f5f5f5;
  font-size: 0.78rem;
  font-weight: 600;
  color: #6b6b6b;
  padding: 10px 14px;
  text-align: left;
  border-bottom: 1px solid #e0e0e0;
}
td {
  font-size: 0.83rem;
  color: #6b7280;
  padding: 10px 14px;
  border-bottom: 1px solid #f0f0f0;
  vertical-align: top;
}
tr:last-child td { border-bottom: none; }
tr:nth-child(even) td { background: #fafafa; }
td:first-child { font-weight: 500; color: #5a5a5a; white-space: nowrap; }

/* ── Solution toggle ───────────────────────────────────── */
.solution-toggle {
  display: inline-flex;
  align-items: center;
  gap: 8px;
  margin: 24px 0 0;
  padding: 10px 20px;
  background: var(--brand-amber);
  color: #1a1a2e;
  border: none;
  border-radius: 22px;
  font-size: 0.84rem;
  font-weight: 700;
  cursor: pointer;
  font-family: inherit;
  transition: background 0.2s, transform 0.1s;
}
.solution-toggle:hover { background: #e0a030; }
.solution-toggle:active { transform: scale(0.98); }
.solution-toggle.open { background: #e0a030; }
.solution-toggle svg {
  transition: transform 0.25s;
}
.solution-toggle.open svg { transform: rotate(180deg); }

.solution-body {
  display: none;
  margin-top: 20px;
  padding: 24px;
  background: #f3f4f6;
  border-radius: 14px;
  border: 1px solid #e5e7eb;
}
.solution-body.visible { display: block; }
.solution-body > .code-wrap:first-child { margin-top: 0; }

/* Grey-toned code blocks inside solution panels */
.solution-body .code-wrap { background: #6b7280; }
.solution-body .code-header { background: #c8c8c8; border-bottom-color: rgba(0,0,0,0.08); }
.solution-body .code-header .code-lang { color: rgba(0,0,0,0.45); }
.solution-body .code-header .code-copy { color: rgba(0,0,0,0.45); }
.solution-body .code-header .code-copy:hover { color: #1a1a2e; background: rgba(0,0,0,0.1); }
.solution-body pre { color: #f9fafb; }
.solution-body pre .kw  { color: #e9b8f5; }
.solution-body pre .fn  { color: #b8d4fa; }
.solution-body pre .num { color: #fcd5b8; }
.solution-body pre .str { color: #b8f0c8; }
.solution-body pre .cm  { color: rgba(255,255,255,0.4); font-style: italic; }
.solution-body pre .tp  { color: #fbb8ba; }

.alt-label {
  font-size: 0.78rem;
  font-weight: 600;
  color: #6b7280;
  text-transform: uppercase;
  letter-spacing: 0.07em;
  margin: 20px 0 6px;
}

/* ── Think About It callout ────────────────────────────── */
.callout {
  background: linear-gradient(135deg, rgba(90,198,210,0.08), rgba(31,69,127,0.06));
  border: 1px solid rgba(90,198,210,0.25);
  border-radius: 14px;
  padding: 22px 24px;
  margin: 28px 0;
}
.callout h3 {
  margin: 0 0 12px;
  font-size: 0.88rem;
  font-weight: 700;
  letter-spacing: 0.04em;
  text-transform: uppercase;
  color: #1a8a95;
}
.callout ul { padding-left: 18px; }
.callout li { color: #374151; font-size: 0.92rem; }

/* ── Chapter intro banner ──────────────────────────────── */
.chapter-banner {
  background: linear-gradient(135deg, var(--brand-navy) 0%, #163460 60%, #0f2347 100%);
  padding: 72px 80px 64px;
  color: white;
  max-width: none;
  border-bottom: none;
}
.chapter-banner .ch-label {
  font-size: 0.72rem;
  font-weight: 700;
  letter-spacing: 0.14em;
  text-transform: uppercase;
  color: var(--brand-amber);
  margin-bottom: 14px;
}
.chapter-banner h1 {
  font-size: 3rem;
  font-weight: 800;
  letter-spacing: -0.04em;
  color: white;
  margin-bottom: 16px;
  line-height: 1.1;
}
.chapter-banner .lead {
  font-size: 1.15rem;
  color: rgba(255,255,255,0.7);
  max-width: 600px;
  line-height: 1.65;
}
.chapter-banner .banner-meta {
  margin-top: 28px;
  display: flex;
  gap: 24px;
  flex-wrap: wrap;
}
.banner-stat {
  display: flex;
  flex-direction: column;
  gap: 2px;
}
.banner-stat .stat-num {
  font-size: 1.6rem;
  font-weight: 700;
  color: var(--brand-amber);
  line-height: 1;
}
.banner-stat .stat-label {
  font-size: 0.75rem;
  color: rgba(255,255,255,0.5);
  text-transform: uppercase;
  letter-spacing: 0.07em;
}

/* ── Index / Home page ─────────────────────────────────── */
.index-hero {
  background: linear-gradient(135deg, var(--brand-navy) 0%, #163460 60%, #0f2347 100%);
  padding: 80px 80px 72px;
  color: white;
}
.index-hero h1 {
  font-size: 3.4rem;
  font-weight: 800;
  letter-spacing: -0.04em;
  color: white;
  margin-bottom: 18px;
  line-height: 1.08;
}
.index-hero .lead {
  font-size: 1.2rem;
  color: rgba(255,255,255,0.72);
  max-width: 580px;
  line-height: 1.65;
  margin-bottom: 36px;
}
.index-hero .hero-badge {
  display: inline-flex;
  align-items: center;
  gap: 8px;
  background: rgba(242,177,62,0.2);
  border: 1px solid rgba(242,177,62,0.4);
  border-radius: 20px;
  padding: 5px 14px;
  font-size: 0.78rem;
  font-weight: 600;
  color: var(--brand-amber);
  letter-spacing: 0.05em;
  text-transform: uppercase;
  margin-bottom: 22px;
}

.chapter-grid {
  max-width: 1100px;
  padding: 64px 80px 100px;
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(300px, 1fr));
  gap: 24px;
}

.chapter-card {
  background: #fff;
  border: 1px solid #e5e7eb;
  border-radius: 18px;
  padding: 28px 28px 24px;
  text-decoration: none;
  color: inherit;
  transition: box-shadow 0.2s, transform 0.2s, border-color 0.2s;
  display: flex;
  flex-direction: column;
  gap: 12px;
}
.chapter-card:hover {
  box-shadow: 0 12px 40px rgba(31,69,127,0.12);
  transform: translateY(-2px);
  border-color: var(--brand-navy);
}
.chapter-card .card-num {
  font-size: 0.7rem;
  font-weight: 700;
  letter-spacing: 0.12em;
  text-transform: uppercase;
  color: rgba(31,69,127,0.4);
}
.chapter-card h2 {
  font-size: 1.25rem;
  font-weight: 700;
  color: #1a1a2e;
  letter-spacing: -0.02em;
  line-height: 1.2;
  border-bottom: none;
  margin: 0;
  padding: 0;
}
.chapter-card p {
  font-size: 0.87rem;
  color: #6b7280;
  line-height: 1.6;
  margin: 0;
}
.chapter-card .card-meta {
  display: flex;
  gap: 14px;
  margin-top: 4px;
}
.card-stat {
  font-size: 0.75rem;
  color: #9ca3af;
}
.card-stat strong {
  color: var(--brand-navy);
  font-weight: 600;
}
.chapter-card .card-arrow {
  margin-top: auto;
  padding-top: 12px;
  font-size: 0.82rem;
  font-weight: 600;
  color: var(--brand-navy);
  display: flex;
  align-items: center;
  gap: 6px;
}

/* Chapter card color accents */
.card-accent {
  width: 36px; height: 4px;
  border-radius: 2px;
  margin-bottom: 4px;
}
.accent-ch1 { background: var(--brand-cyan); }
.accent-ch2 { background: var(--brand-amber); }
.accent-ch3 { background: var(--brand-green); }
.accent-ch4 { background: var(--brand-coral); }
.accent-ch5 { background: #a78bfa; }
.accent-ch6 { background: #fb923c; }

/* Index page overall progress */
.overall-progress {
  max-width: 1100px;
  padding: 0 80px 48px;
}
.overall-progress h2 {
  font-size: 1rem;
  font-weight: 600;
  color: #374151;
  margin-bottom: 16px;
  letter-spacing: -0.01em;
  border: none;
  padding: 0;
}
.overall-progress-bar-wrap {
  height: 6px;
  background: #e5e7eb;
  border-radius: 3px;
  overflow: hidden;
  margin-bottom: 8px;
}
.overall-progress-bar-fill {
  height: 100%;
  background: linear-gradient(90deg, var(--brand-navy), var(--brand-cyan));
  border-radius: 3px;
  transition: width 0.5s ease;
}
.overall-progress-label {
  font-size: 0.78rem;
  color: #9ca3af;
}

/* ── Responsive ────────────────────────────────────────── */
@media (max-width: 900px) {
  .sidebar { display: none; }
  .section-hero, .article { padding-left: 28px; padding-right: 28px; }
  .chapter-banner { padding: 48px 28px 40px; }
  .chapter-banner h1 { font-size: 2rem; }
  .index-hero { padding: 48px 28px 40px; }
  .index-hero h1 { font-size: 2.2rem; }
  .chapter-grid { padding: 40px 28px 60px; }
  .overall-progress { padding: 0 28px 40px; }
}
@media (max-width: 600px) {
  .topbar-logo span { display: none; }
  .nav-pill-btn .pill-label { display: none; }
  .chapter-grid { grid-template-columns: 1fr; }
}

/* ── Chapter footer nav ─────────────────────────────────── */
.chapter-footer {
  display: flex;
  justify-content: flex-end;
  padding: 40px 20px 60px;
}

.btn-next-chapter {
  display: inline-flex;
  align-items: center;
  gap: 8px;
  background: var(--brand-amber, #f5a623);
  color: #1d1d1f;
  font-weight: 700;
  font-size: 1rem;
  padding: 12px 24px;
  border-radius: 980px;
  text-decoration: none;
  transition: background 0.15s, transform 0.15s;
}

.btn-next-chapter:hover {
  background: #e09400;
  transform: translateX(3px);
}

.next-arrow {
  font-size: 1.1em;
}