/* ═══════════════════════════════════════════════════════════════════════
 * QuantLesson — Premium Dark Theme Stylesheet
 * ═══════════════════════════════════════════════════════════════════════ */

/* ── CSS Custom Properties ─────────────────────────────────────────── */
:root {
  /* Backgrounds */
  --bg-primary:    #0d1117;
  --bg-secondary:  #161b22;
  --bg-tertiary:   #1c2128;
  --bg-hover:      #21262d;

  /* Borders */
  --border-color:  #30363d;

  /* Text */
  --text-primary:   #f0f6fc;
  --text-secondary: #c9d1d9;
  --text-muted:     #8b949e;

  /* Accents */
  --accent-blue:    #58a6ff;
  --accent-green:   #7ee787;
  --accent-orange:  #d29922;
  --accent-red:     #f85149;
  --accent-purple:  #bc8cff;

  /* Gradients */
  --gradient-start: #1f6feb;
  --gradient-end:   #58a6ff;

  /* Layout */
  --sidebar-width:  280px;
  --header-height:  60px;

  /* Typography */
  --font-body: 'Inter', -apple-system, BlinkMacSystemFont, 'Segoe UI', Tahoma, sans-serif;
  --font-mono: 'JetBrains Mono', 'Fira Code', 'Consolas', monospace;
}

/* ── Reset / Base ──────────────────────────────────────────────────── */
*,
*::before,
*::after {
  box-sizing: border-box;
  margin: 0;
  padding: 0;
}

html {
  scroll-behavior: smooth;
  font-size: 16px;
}

body {
  font-family: var(--font-body);
  background: var(--bg-primary);
  color: var(--text-secondary);
  line-height: 1.6;
  min-height: 100vh;
  overflow-x: hidden;
  -webkit-font-smoothing: antialiased;
  -moz-osx-font-smoothing: grayscale;
}

a {
  color: var(--accent-blue);
  text-decoration: none;
  transition: color 0.2s ease;
}

a:hover {
  color: var(--accent-purple);
}

img {
  max-width: 100%;
  height: auto;
}

/* ── Scrollbar (WebKit) ────────────────────────────────────────────── */
::-webkit-scrollbar {
  width: 8px;
  height: 8px;
}

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

::-webkit-scrollbar-thumb {
  background: var(--border-color);
  border-radius: 4px;
}

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

/* Firefox */
* {
  scrollbar-width: thin;
  scrollbar-color: var(--border-color) var(--bg-primary);
}

/* ══════════════════════════════════════════════════════════════════════
 * 1. HEADER
 * ══════════════════════════════════════════════════════════════════════ */
.header {
  position: sticky;
  top: 0;
  z-index: 100;
  display: flex;
  align-items: center;
  justify-content: space-between;
  height: var(--header-height);
  padding: 0 1.25rem;
  background: linear-gradient(135deg, var(--gradient-start), var(--gradient-end));
  backdrop-filter: blur(12px);
  -webkit-backdrop-filter: blur(12px);
  box-shadow: 0 2px 16px rgba(0, 0, 0, 0.4);
}

.header-left {
  display: flex;
  align-items: center;
  gap: 0.75rem;
}

.hamburger {
  display: none;            /* shown only on tablet/mobile */
  background: none;
  border: none;
  color: #fff;
  font-size: 1.4rem;
  cursor: pointer;
  padding: 0.25rem 0.5rem;
  border-radius: 4px;
  transition: background 0.2s;
}

.hamburger:hover {
  background: rgba(255, 255, 255, 0.15);
}

.logo {
  color: #fff;
  font-size: 1.1rem;
  font-weight: 700;
  letter-spacing: 0.08em;
  text-transform: uppercase;
}

.logo:hover {
  color: #fff;
  opacity: 0.9;
}

.header-right {
  display: flex;
  align-items: center;
  gap: 1rem;
  font-size: 0.875rem;
}

.user-info {
  color: rgba(255, 255, 255, 0.9);
}

.header-link {
  color: rgba(255, 255, 255, 0.85);
  padding: 0.3rem 0.65rem;
  border-radius: 6px;
  font-weight: 500;
  transition: background 0.2s, color 0.2s;
}

.header-link:hover {
  background: rgba(255, 255, 255, 0.15);
  color: #fff;
}

.logout-link:hover {
  background: rgba(248, 81, 73, 0.25);
  color: var(--accent-red);
}

/* ══════════════════════════════════════════════════════════════════════
 * 2. APP CONTAINER (grid)
 * ══════════════════════════════════════════════════════════════════════ */
.app-container {
  display: grid;
  grid-template-columns: var(--sidebar-width) 1fr;
  min-height: calc(100vh - var(--header-height));
}

/* ══════════════════════════════════════════════════════════════════════
 * 3. SIDEBAR
 * ══════════════════════════════════════════════════════════════════════ */
.sidebar {
  position: fixed;
  top: var(--header-height);
  left: 0;
  width: var(--sidebar-width);
  height: calc(100vh - var(--header-height));
  background: var(--bg-secondary);
  border-right: 1px solid var(--border-color);
  overflow-y: auto;
  z-index: 50;
  transition: transform 0.3s ease;
}

.sidebar-header {
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 1.25rem 1rem 0.75rem;
  border-bottom: 1px solid var(--border-color);
}

.sidebar-header h2 {
  font-size: 0.95rem;
  font-weight: 600;
  color: var(--text-primary);
  letter-spacing: 0.04em;
}

.sidebar-close {
  display: none;           /* shown on mobile */
  background: none;
  border: none;
  color: var(--text-muted);
  font-size: 1.5rem;
  cursor: pointer;
  padding: 0 0.3rem;
  line-height: 1;
  transition: color 0.2s;
}

.sidebar-close:hover {
  color: var(--text-primary);
}

.sidebar-nav {
  padding: 0.75rem 0 2rem;
}

/* Part groupings */
.toc-part {
  margin-bottom: 0.5rem;
}

.toc-part-title {
  padding: 0.6rem 1rem 0.35rem;
  font-size: 0.7rem;
  font-weight: 700;
  letter-spacing: 0.1em;
  text-transform: uppercase;
  color: var(--text-muted);
  cursor: pointer;
  user-select: none;
  transition: color 0.2s;
}

.toc-part-title:hover {
  color: var(--accent-blue);
}

/* Chapter entries */
.toc-chapter {
  position: relative;
}

.toc-link {
  display: block;
  padding: 0.45rem 1rem 0.45rem 1.25rem;
  font-size: 0.82rem;
  color: var(--text-secondary);
  border-left: 3px solid transparent;
  transition: background 0.15s, border-color 0.15s, color 0.15s;
  line-height: 1.45;
}

.toc-link:hover {
  background: var(--bg-hover);
  color: var(--text-primary);
}

.toc-link.active {
  color: var(--accent-blue);
  border-left-color: var(--accent-blue);
  background: rgba(88, 166, 255, 0.08);
  font-weight: 500;
}

/* Sub-chapters (collapsible) */
.toc-subchapters {
  display: none;
  overflow: hidden;
  transition: max-height 0.3s ease;
}

.toc-chapter.expanded .toc-subchapters {
  display: block;
}

.toc-sub {
  padding-left: 2rem;
  font-size: 0.78rem;
}

/* Sidebar overlay (mobile) */
.sidebar-overlay {
  display: none;
  position: fixed;
  inset: 0;
  background: rgba(0, 0, 0, 0.55);
  z-index: 40;
  backdrop-filter: blur(2px);
  -webkit-backdrop-filter: blur(2px);
}

/* ══════════════════════════════════════════════════════════════════════
 * 4. MAIN CONTENT AREA
 * ══════════════════════════════════════════════════════════════════════ */
.main-content {
  max-width: 960px;
  width: 100%;
  margin: 0 auto;
  padding: 2rem 2.5rem 4rem;
}

/* ══════════════════════════════════════════════════════════════════════
 * 5. BREADCRUMB
 * ══════════════════════════════════════════════════════════════════════ */
.breadcrumb {
  display: flex;
  align-items: center;
  gap: 0.4rem;
  font-size: 0.8rem;
  color: var(--text-muted);
  margin-bottom: 1.25rem;
}

.breadcrumb-sep {
  color: var(--accent-blue);
}

/* ══════════════════════════════════════════════════════════════════════
 * 6. CHAPTER CONTENT
 * ══════════════════════════════════════════════════════════════════════ */
.chapter-page {
  /* wrapper */
}

.chapter-content {
  margin: 2rem 0;
}

.chapter-title {
  font-size: 1.85rem;
  font-weight: 700;
  line-height: 1.3;
  margin-bottom: 2rem;
  color: var(--text-primary);
}

.content-body {
  font-size: 1rem;
  line-height: 1.7;
  color: var(--text-secondary);
  overflow-wrap: break-word;
}

/* Headings inside content */
.content-body h2 {
  font-size: 1.4rem;
  font-weight: 600;
  color: var(--text-primary);
  margin: 2.5rem 0 1rem;
  padding-bottom: 0.5rem;
  border-bottom: 2px solid var(--accent-blue);
}

.content-body h3 {
  font-size: 1.15rem;
  font-weight: 600;
  color: var(--text-primary);
  margin: 2rem 0 0.75rem;
  padding-left: 0.75rem;
  border-left: 3px solid var(--accent-purple);
}

.content-body h4 {
  font-size: 1.05rem;
  font-weight: 600;
  color: var(--text-primary);
  margin: 1.5rem 0 0.5rem;
}

.content-body p {
  margin-bottom: 1rem;
}

.content-body ul,
.content-body ol {
  margin: 0.75rem 0 1.25rem 1.5rem;
}

.content-body li {
  margin-bottom: 0.35rem;
  line-height: 1.65;
}

.content-body ul li::marker {
  color: var(--accent-blue);
}

.content-body ol li::marker {
  color: var(--accent-orange);
  font-weight: 600;
}

.content-body blockquote {
  margin: 1.25rem 0;
  padding: 0.75rem 1.25rem;
  border-left: 4px solid var(--accent-purple);
  background: rgba(188, 140, 255, 0.06);
  font-style: italic;
  color: var(--text-muted);
  border-radius: 0 6px 6px 0;
}

.content-body code {
  font-family: var(--font-mono);
  font-size: 0.88em;
  background: var(--bg-secondary);
  color: var(--accent-orange);
  padding: 0.15em 0.4em;
  border-radius: 4px;
}

.content-body pre {
  margin: 1.25rem 0;
  padding: 1.15rem 1.25rem;
  background: var(--bg-secondary);
  border: 1px solid var(--border-color);
  border-radius: 8px;
  overflow-x: auto;
  font-size: 0.88rem;
  line-height: 1.55;
}

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

.content-body hr {
  border: none;
  height: 1px;
  background: linear-gradient(
    90deg,
    transparent,
    var(--border-color) 20%,
    var(--border-color) 80%,
    transparent
  );
  margin: 2rem 0;
}

.content-body strong {
  color: var(--text-primary);
  font-weight: 600;
}

.content-body em {
  color: var(--accent-purple);
}

/* ── Content formatting classes (formatContent output) ──────────────── */

/* ════ Main header */
.content-header-main {
  font-size: 1.5rem;
  font-weight: 700;
  color: var(--text-primary);
  text-align: center;
  margin: 2.5rem 0 1.5rem;
  padding: 1rem 0;
  border-top: 2px solid var(--accent-blue);
  border-bottom: 2px solid var(--accent-blue);
  letter-spacing: 0.03em;
}

/* ──── Section header */
.content-header-section {
  font-size: 1.25rem;
  font-weight: 600;
  color: var(--accent-blue);
  margin: 2rem 0 1rem;
  padding-bottom: 0.4rem;
  border-bottom: 1px solid var(--border-color);
}

/* ■ Sub header */
.content-header-sub {
  font-size: 1.1rem;
  font-weight: 600;
  color: var(--accent-purple);
  margin: 1.5rem 0 0.75rem;
  padding-left: 0.6rem;
  border-left: 3px solid var(--accent-purple);
}

/* Bullet list */
.content-list {
  list-style: none;
  margin: 0.75rem 0 1.25rem 0;
  padding: 0;
}

.content-list li {
  position: relative;
  padding-left: 1.5rem;
  margin-bottom: 0.4rem;
  line-height: 1.65;
}

.content-list li::before {
  content: '▸';
  position: absolute;
  left: 0;
  color: var(--accent-blue);
  font-weight: 700;
}

/* Numbered list */
.content-numbered-list {
  margin: 0.75rem 0 1.25rem 1.5rem;
  padding: 0;
}

.content-numbered-list li {
  margin-bottom: 0.4rem;
  line-height: 1.65;
}

.content-numbered-list li::marker {
  color: var(--accent-orange);
  font-weight: 700;
}

/* Code block */
.content-code {
  margin: 1.25rem 0;
  padding: 1.15rem 1.25rem;
  background: var(--bg-secondary);
  border: 1px solid var(--border-color);
  border-radius: 8px;
  font-family: var(--font-mono);
  font-size: 0.88rem;
  line-height: 1.55;
  overflow-x: auto;
  color: var(--text-secondary);
  white-space: pre-wrap;
  word-break: break-word;
}

/* Summary box */
.content-summary {
  margin: 1.5rem 0;
  padding: 1.25rem 1.5rem;
  background: rgba(88, 166, 255, 0.06);
  border: 1px solid rgba(88, 166, 255, 0.2);
  border-radius: 8px;
  border-left: 4px solid var(--accent-blue);
}

.content-summary p:last-child {
  margin-bottom: 0;
}

/* Vocabulary box */
.content-vocab {
  margin: 1.5rem 0;
  padding: 1.25rem 1.5rem;
  background: rgba(188, 140, 255, 0.06);
  border: 1px solid rgba(188, 140, 255, 0.15);
  border-radius: 8px;
  border-left: 4px solid var(--accent-purple);
}

/* Review questions */
.content-question {
  margin: 1.5rem 0;
  padding: 1.25rem 1.5rem;
  background: rgba(126, 231, 135, 0.06);
  border: 1px solid rgba(126, 231, 135, 0.15);
  border-radius: 8px;
  border-left: 4px solid var(--accent-green);
}

/* ══════════════════════════════════════════════════════════════════════
 * 7. CHAPTER NAVIGATION
 * ══════════════════════════════════════════════════════════════════════ */
.chapter-nav {
  display: flex;
  justify-content: space-between;
  gap: 1rem;
}

.nav-btn {
  display: flex;
  flex-direction: column;
  flex: 1;
  padding: 1rem 1.25rem;
  background: var(--bg-tertiary);
  border: 1px solid var(--border-color);
  border-radius: 8px;
  transition: transform 0.2s ease, box-shadow 0.2s ease, border-color 0.2s ease;
  text-decoration: none;
}

.nav-btn:hover {
  transform: translateY(-2px);
  box-shadow: 0 4px 16px rgba(0, 0, 0, 0.3);
  border-color: var(--accent-blue);
  color: inherit;
}

.nav-btn.disabled {
  opacity: 0.35;
  pointer-events: none;
  cursor: default;
}

.nav-prev {
  align-items: flex-start;
}

.nav-next {
  align-items: flex-end;
  text-align: right;
}

.nav-direction {
  font-size: 0.75rem;
  color: var(--text-muted);
  margin-bottom: 0.25rem;
  font-weight: 500;
  text-transform: uppercase;
  letter-spacing: 0.04em;
}

.nav-title {
  font-size: 0.9rem;
  color: var(--accent-blue);
  font-weight: 500;
  line-height: 1.4;
}

/* ══════════════════════════════════════════════════════════════════════
 * 8. NOTES SECTION
 * ══════════════════════════════════════════════════════════════════════ */
.notes-section {
  margin-top: 3rem;
  padding: 1.5rem;
  background: var(--bg-tertiary);
  border: 1px solid var(--border-color);
  border-radius: 8px;
}

.notes-header {
  display: flex;
  align-items: center;
  justify-content: space-between;
  margin-bottom: 0.85rem;
}

.notes-header h3 {
  font-size: 1rem;
  font-weight: 600;
  color: var(--text-primary);
}

.save-status {
  font-size: 0.8rem;
  color: var(--text-muted);
  transition: opacity 0.3s ease;
}

.save-status.saving {
  color: var(--accent-orange);
}

.save-status.saved {
  color: var(--accent-green);
}

.save-status.error {
  color: var(--accent-red);
}

.save-status.fade-out {
  opacity: 0;
}

#noteTextarea {
  width: 100%;
  min-height: 150px;
  padding: 1rem;
  background: var(--bg-primary);
  color: var(--text-secondary);
  border: 1px solid var(--border-color);
  border-radius: 6px;
  font-family: var(--font-body);
  font-size: 0.92rem;
  line-height: 1.6;
  resize: vertical;
  transition: border-color 0.2s ease, box-shadow 0.2s ease;
}

#noteTextarea:focus {
  outline: none;
  border-color: var(--accent-blue);
  box-shadow: 0 0 0 3px rgba(88, 166, 255, 0.15);
}

#noteTextarea::placeholder {
  color: var(--text-muted);
}

/* ══════════════════════════════════════════════════════════════════════
 * 9. LOGIN PAGE
 * ══════════════════════════════════════════════════════════════════════ */
.login-page {
  display: flex;
  align-items: center;
  justify-content: center;
  min-height: 100vh;
  background: var(--bg-primary);
  background-image:
    radial-gradient(ellipse at 20% 50%, rgba(31, 111, 235, 0.12) 0%, transparent 60%),
    radial-gradient(ellipse at 80% 50%, rgba(188, 140, 255, 0.08) 0%, transparent 60%);
  animation: bgShift 12s ease-in-out infinite alternate;
}

@keyframes bgShift {
  0%   { background-position: 0% 50%; }
  100% { background-position: 100% 50%; }
}

.login-container {
  width: 100%;
  max-width: 420px;
  padding: 1.5rem;
}

.login-card {
  background: rgba(22, 27, 34, 0.85);
  backdrop-filter: blur(16px);
  -webkit-backdrop-filter: blur(16px);
  border: 1px solid var(--border-color);
  border-radius: 16px;
  padding: 2.5rem 2rem;
  text-align: center;
  box-shadow: 0 16px 48px rgba(0, 0, 0, 0.4);
}

.login-logo {
  font-size: 3rem;
  color: var(--accent-blue);
  margin-bottom: 0.5rem;
  animation: pulse 3s ease-in-out infinite;
}

@keyframes pulse {
  0%, 100% { transform: scale(1); opacity: 1; }
  50%      { transform: scale(1.06); opacity: 0.85; }
}

.login-card h1 {
  font-size: 1.6rem;
  font-weight: 700;
  color: var(--text-primary);
  margin-bottom: 0.15rem;
}

.login-card h2 {
  font-size: 0.95rem;
  font-weight: 400;
  color: var(--text-muted);
  margin-bottom: 1.75rem;
}

.login-card .form-group {
  margin-bottom: 1.15rem;
  text-align: left;
}

.login-card label {
  display: block;
  font-size: 0.82rem;
  font-weight: 500;
  color: var(--text-muted);
  margin-bottom: 0.4rem;
}

.login-card input[type="text"],
.login-card input[type="password"] {
  width: 100%;
  padding: 0.7rem 0.9rem;
  background: var(--bg-primary);
  border: 1px solid var(--border-color);
  border-radius: 8px;
  color: var(--text-primary);
  font-size: 0.95rem;
  font-family: var(--font-body);
  transition: border-color 0.2s, box-shadow 0.2s;
}

.login-card input:focus {
  outline: none;
  border-color: var(--accent-blue);
  box-shadow: 0 0 0 3px rgba(88, 166, 255, 0.18);
}

.btn-primary {
  display: block;
  width: 100%;
  padding: 0.75rem;
  margin-top: 0.5rem;
  background: linear-gradient(135deg, var(--gradient-start), var(--gradient-end));
  color: #fff;
  font-size: 1rem;
  font-weight: 600;
  border: none;
  border-radius: 8px;
  cursor: pointer;
  transition: transform 0.15s, box-shadow 0.2s;
  font-family: var(--font-body);
}

.btn-primary:hover {
  transform: scale(1.02);
  box-shadow: 0 4px 20px rgba(31, 111, 235, 0.4);
}

.btn-primary:active {
  transform: scale(0.98);
}

/* ── Alerts ─────────────────────────────────────────────────────────── */
.alert {
  padding: 0.75rem 1rem;
  border-radius: 8px;
  font-size: 0.88rem;
  margin-bottom: 1.25rem;
}

.alert-error {
  background: rgba(248, 81, 73, 0.1);
  border: 1px solid rgba(248, 81, 73, 0.3);
  color: var(--accent-red);
}

.alert-success {
  background: rgba(126, 231, 135, 0.1);
  border: 1px solid rgba(126, 231, 135, 0.25);
  color: var(--accent-green);
}

/* ══════════════════════════════════════════════════════════════════════
 * 10. ADMIN PAGE
 * ══════════════════════════════════════════════════════════════════════ */
.admin-page {
  max-width: 900px;
}

.admin-title {
  font-size: 1.65rem;
  font-weight: 700;
  color: var(--text-primary);
  margin-bottom: 1.75rem;
}

.admin-section {
  margin-bottom: 2.5rem;
}

.admin-section h2 {
  font-size: 1.15rem;
  font-weight: 600;
  color: var(--text-primary);
  margin-bottom: 1rem;
  padding-bottom: 0.4rem;
  border-bottom: 1px solid var(--border-color);
}

/* Admin create-user form */
.admin-form .form-row {
  display: flex;
  gap: 0.85rem;
  flex-wrap: wrap;
  align-items: flex-end;
}

.admin-form .form-group {
  flex: 1;
  min-width: 160px;
}

.admin-form .form-group-btn {
  flex: 0 0 auto;
  min-width: auto;
}

.admin-form label {
  display: block;
  font-size: 0.8rem;
  font-weight: 500;
  color: var(--text-muted);
  margin-bottom: 0.35rem;
}

.admin-form input[type="text"],
.admin-form input[type="password"] {
  width: 100%;
  padding: 0.6rem 0.8rem;
  background: var(--bg-primary);
  border: 1px solid var(--border-color);
  border-radius: 6px;
  color: var(--text-primary);
  font-size: 0.9rem;
  font-family: var(--font-body);
  transition: border-color 0.2s, box-shadow 0.2s;
}

.admin-form input:focus {
  outline: none;
  border-color: var(--accent-blue);
  box-shadow: 0 0 0 3px rgba(88, 166, 255, 0.15);
}

/* Admin users table */
.table-responsive {
  overflow-x: auto;
}

.admin-table {
  width: 100%;
  border-collapse: collapse;
  font-size: 0.88rem;
}

.admin-table th {
  text-align: left;
  padding: 0.65rem 0.85rem;
  background: var(--bg-secondary);
  color: var(--text-muted);
  font-weight: 600;
  font-size: 0.78rem;
  text-transform: uppercase;
  letter-spacing: 0.05em;
  border-bottom: 2px solid var(--border-color);
}

.admin-table td {
  padding: 0.65rem 0.85rem;
  border-bottom: 1px solid var(--border-color);
  color: var(--text-secondary);
}

.admin-table tbody tr:nth-child(even) {
  background: rgba(22, 27, 34, 0.5);
}

.admin-table tbody tr:hover {
  background: var(--bg-hover);
}

.badge {
  display: inline-block;
  padding: 0.15rem 0.55rem;
  border-radius: 12px;
  font-size: 0.72rem;
  font-weight: 600;
  text-transform: uppercase;
  letter-spacing: 0.04em;
}

.badge-admin {
  background: rgba(88, 166, 255, 0.15);
  color: var(--accent-blue);
}

.badge-user {
  background: rgba(139, 148, 158, 0.15);
  color: var(--text-muted);
}

.btn-danger {
  background: rgba(248, 81, 73, 0.12);
  color: var(--accent-red);
  border: 1px solid rgba(248, 81, 73, 0.3);
  padding: 0.3rem 0.75rem;
  border-radius: 6px;
  font-size: 0.8rem;
  font-weight: 500;
  cursor: pointer;
  font-family: var(--font-body);
  transition: background 0.2s, border-color 0.2s;
}

.btn-danger:hover {
  background: rgba(248, 81, 73, 0.25);
  border-color: var(--accent-red);
}

.btn-sm {
  padding: 0.25rem 0.6rem;
  font-size: 0.78rem;
}

.inline-form {
  display: inline;
}

.text-center {
  text-align: center;
}

.text-muted {
  color: var(--text-muted);
}

/* ══════════════════════════════════════════════════════════════════════
 * 11. SCROLL-TO-TOP BUTTON
 * ══════════════════════════════════════════════════════════════════════ */
.scroll-top-btn {
  position: fixed;
  bottom: 2rem;
  right: 2rem;
  width: 42px;
  height: 42px;
  background: var(--bg-tertiary);
  border: 1px solid var(--border-color);
  border-radius: 50%;
  color: var(--text-muted);
  font-size: 1.15rem;
  cursor: pointer;
  opacity: 0;
  visibility: hidden;
  transition: opacity 0.25s, visibility 0.25s, transform 0.2s, background 0.2s;
  z-index: 30;
  display: flex;
  align-items: center;
  justify-content: center;
}

.scroll-top-btn.visible {
  opacity: 1;
  visibility: visible;
}

.scroll-top-btn:hover {
  background: var(--accent-blue);
  color: #fff;
  border-color: var(--accent-blue);
  transform: translateY(-2px);
}

/* ══════════════════════════════════════════════════════════════════════
 * 12. RESPONSIVE DESIGN
 * ══════════════════════════════════════════════════════════════════════ */

/* Tablet (≤1024px) — sidebar becomes overlay */
@media (max-width: 1024px) {
  .app-container {
    grid-template-columns: 1fr;
  }

  .sidebar {
    transform: translateX(-100%);
  }

  .sidebar-close {
    display: block;
  }

  .hamburger {
    display: block;
  }

  /* When body has sidebar-open class */
  body.sidebar-open .sidebar {
    transform: translateX(0);
  }

  body.sidebar-open .sidebar-overlay {
    display: block;
  }

  .main-content {
    padding: 1.5rem 1.75rem 3rem;
  }
}

/* Mobile (≤768px) */
@media (max-width: 768px) {
  .header {
    padding: 0 0.75rem;
  }

  .header-right {
    gap: 0.5rem;
    font-size: 0.8rem;
  }

  .user-info {
    display: none;
  }

  .main-content {
    padding: 1.25rem 1rem 2.5rem;
  }

  .chapter-title {
    font-size: 1.4rem;
  }

  .chapter-nav {
    flex-direction: column;
  }

  .nav-next {
    align-items: flex-start;
    text-align: left;
  }

  .admin-form .form-row {
    flex-direction: column;
  }

  .admin-form .form-group {
    min-width: 100%;
  }

  .login-card {
    padding: 2rem 1.5rem;
  }

  .scroll-top-btn {
    bottom: 1.25rem;
    right: 1.25rem;
    width: 38px;
    height: 38px;
  }
}

/* Small mobile (≤480px) */
@media (max-width: 480px) {
  .main-content {
    padding: 1rem 0.75rem 2rem;
  }

  .chapter-title {
    font-size: 1.2rem;
  }

  .content-body {
    font-size: 0.92rem;
  }

  .nav-btn {
    padding: 0.75rem 1rem;
  }

  .notes-section {
    padding: 1rem;
  }

  .header-link {
    padding: 0.25rem 0.4rem;
    font-size: 0.78rem;
  }
}