:root {
  --sidebar-w: 340px;
  --topbar-h: 54px;
  --bg: #f8f6f1;
  --bg-card: #ffffff;
  --sidebar-bg: #16181f;
  --sidebar-hover: #1e2130;
  --sidebar-active: #252838;
  --sidebar-text: #b8bcd0;
  --sidebar-text-dim: #60637a;
  --sidebar-border: #252838;
  --text: #1e1e1e;
  --text-muted: #767676;
  --text-light: #a0a0a0;
  --border: #e8e5dc;
  --shadow-md: 0 4px 16px rgba(0, 0, 0, 0.1);
  --radius: 8px;
  --radius-lg: 12px;
  --font-body: Inter, system-ui, -apple-system, BlinkMacSystemFont, "Segoe UI", sans-serif;
  --font-heading: Lora, Georgia, "Times New Roman", serif;
  --transition: 200ms ease;
}

*, *::before, *::after { box-sizing: border-box; margin: 0; padding: 0; }
html { scroll-behavior: smooth; }
body {
  font-family: var(--font-body);
  font-size: 16px;
  line-height: 1.65;
  color: var(--text);
  background: var(--bg);
  overflow-x: hidden;
}
a { color: inherit; text-decoration: none; }
button { font: inherit; }

.app { display: flex; min-height: 100vh; padding-top: var(--topbar-h); }

.topbar {
  position: fixed;
  inset: 0 0 auto 0;
  height: var(--topbar-h);
  z-index: 300;
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 16px;
  padding: 0 22px;
  background: rgba(248, 246, 241, .94);
  border-bottom: 1px solid var(--border);
  backdrop-filter: blur(14px);
}
.topbar-brand {
  font-size: 13px;
  font-weight: 800;
  letter-spacing: .08em;
  text-transform: uppercase;
  color: var(--text);
}

.sidebar {
  width: var(--sidebar-w);
  min-height: 100vh;
  background: var(--sidebar-bg);
  display: flex;
  flex-direction: column;
  position: fixed;
  inset: var(--topbar-h) auto 0 0;
  z-index: 100;
  overflow-y: auto;
  overflow-x: hidden;
  transition: transform var(--transition);
  scrollbar-width: thin;
  scrollbar-color: #2e3248 transparent;
}
.sidebar::-webkit-scrollbar { width: 4px; }
.sidebar::-webkit-scrollbar-track { background: transparent; }
.sidebar::-webkit-scrollbar-thumb { background: #2e3248; border-radius: 4px; }

.sidebar-header {
  padding: 24px 20px 18px;
  border-bottom: 1px solid var(--sidebar-border);
}
.course-badge {
  display: inline-block;
  font-size: 11px;
  font-weight: 700;
  letter-spacing: .08em;
  text-transform: uppercase;
  color: var(--sidebar-text-dim);
  border: 1px solid #2e3248;
  border-radius: 4px;
  padding: 3px 8px;
  margin-bottom: 10px;
}
.course-institution {
  font-size: 10px;
  letter-spacing: .06em;
  text-transform: uppercase;
  color: var(--sidebar-text-dim);
  margin-bottom: 6px;
}
.course-title {
  font-family: var(--font-heading);
  font-size: 17px;
  font-weight: 700;
  color: #e8eaf0;
  line-height: 1.3;
  margin-bottom: 2px;
}
.course-subtitle {
  font-size: 12px;
  color: var(--sidebar-text-dim);
  font-style: italic;
}
.language-toggle {
  border: 1px solid var(--border);
  border-radius: 999px;
  background: var(--bg-card);
  color: var(--text);
  padding: 6px 12px;
  cursor: pointer;
  font-size: 13px;
  font-weight: 700;
}
.language-toggle:hover { border-color: #c9c2b6; }

.sidebar-nav { padding: 12px 0 24px; }
.nav-course { margin-bottom: 5px; }
.nav-course-header {
  width: 100%;
  display: flex;
  align-items: center;
  gap: 11px;
  padding: 12px 16px;
  cursor: pointer;
  border: 0;
  border-left: 3px solid transparent;
  background: transparent;
  color: inherit;
  text-align: left;
  transition: background var(--transition);
}
.nav-course-header:hover { background: var(--sidebar-hover); }
.nav-course-header.active-course { border-left-color: var(--course-color, #888); }
.nav-course-order {
  width: 28px;
  height: 28px;
  display: inline-flex;
  align-items: center;
  justify-content: center;
  border-radius: 999px;
  background: #222638;
  color: #d8dcf0;
  font-size: 12px;
  font-weight: 800;
  flex-shrink: 0;
}
.active-course .nav-course-order {
  background: var(--course-color, #888);
  color: #fff;
}
.nav-course-info { flex: 1; min-width: 0; display: block; }
.nav-course-number {
  display: block;
  font-size: 10px;
  font-weight: 800;
  letter-spacing: .07em;
  text-transform: uppercase;
  color: #8890b0;
  margin-bottom: 1px;
}
.nav-course-title {
  display: block;
  font-size: 14px;
  font-weight: 800;
  color: #eef0fb;
  line-height: 1.25;
}
.nav-course-duration {
  display: block;
  font-size: 11px;
  color: var(--sidebar-text-dim);
  margin-top: 1px;
}
.nav-course-progress {
  display: block;
  width: 100%;
  height: 2px;
  background: #2a2d3e;
  border-radius: 1px;
  margin-top: 6px;
  overflow: hidden;
}
.nav-course-progress-fill {
  display: block;
  height: 100%;
  border-radius: 1px;
  background: var(--course-color, #888);
}
.nav-course-chevron {
  font-size: 11px;
  color: #8890b0;
  transition: transform var(--transition);
  flex-shrink: 0;
}
.nav-course.open .nav-course-chevron { transform: rotate(90deg); }
.nav-course-content { display: none; padding: 2px 0 6px; }
.nav-course.open .nav-course-content { display: block; }
.nav-empty {
  color: var(--sidebar-text-dim);
  font-size: 12px;
  padding: 8px 20px 10px 58px;
}
.nav-module { margin-bottom: 4px; }
.nav-module-header {
  width: 100%;
  display: flex;
  align-items: center;
  gap: 10px;
  padding: 9px 20px 9px 38px;
  cursor: pointer;
  border: 0;
  border-left: 3px solid transparent;
  background: transparent;
  color: inherit;
  text-align: left;
  transition: background var(--transition);
}
.nav-module-header:hover { background: var(--sidebar-hover); }
.nav-module-header.active-module { border-left-color: var(--module-color, #888); }
.nav-module-icon { font-size: 16px; width: 20px; text-align: center; flex-shrink: 0; color: #d8dcf0; }
.nav-module-info { flex: 1; min-width: 0; display: block; }
.nav-module-number {
  display: block;
  font-size: 10px;
  font-weight: 700;
  letter-spacing: .07em;
  text-transform: uppercase;
  color: #8890b0;
  margin-bottom: 2px;
}
.nav-module-title {
  display: block;
  font-size: 13px;
  font-weight: 700;
  color: #d8dcf0;
  white-space: nowrap;
  overflow: hidden;
  text-overflow: ellipsis;
  line-height: 1.3;
}
.nav-module-chevron {
  font-size: 11px;
  color: #8890b0;
  transition: transform var(--transition);
  flex-shrink: 0;
}
.nav-module.open .nav-module-chevron { transform: rotate(90deg); }
.nav-module-progress {
  display: block;
  width: 100%;
  height: 2px;
  background: #2a2d3e;
  border-radius: 1px;
  margin-top: 5px;
  overflow: hidden;
}
.nav-module-progress-fill {
  display: block;
  height: 100%;
  border-radius: 1px;
  background: var(--module-color, #888);
}
.nav-lessons { display: none; padding: 2px 0 6px; }
.nav-module.open .nav-lessons { display: block; }
.nav-lesson {
  width: 100%;
  display: flex;
  align-items: center;
  gap: 8px;
  padding: 8px 20px 8px 64px;
  cursor: pointer;
  border: 0;
  border-left: 3px solid transparent;
  background: transparent;
  color: inherit;
  text-align: left;
  transition: background var(--transition);
}
.nav-lesson:hover { background: var(--sidebar-hover); }
.nav-lesson.active { background: var(--sidebar-active); border-left-color: var(--module-color, #888); }
.nav-lesson-dot {
  width: 6px;
  height: 6px;
  border-radius: 50%;
  border: 1.5px solid var(--sidebar-text-dim);
  flex-shrink: 0;
}
.nav-lesson.has-content .nav-lesson-dot,
.nav-lesson.active .nav-lesson-dot {
  background: var(--module-color, #888);
  border-color: var(--module-color, #888);
}
.nav-lesson-id {
  font-size: 11px;
  font-weight: 700;
  color: #8890b0;
  min-width: 28px;
}
.nav-lesson-title {
  font-size: 13px;
  color: #c4c8e0;
  line-height: 1.35;
}
.nav-lesson.active .nav-lesson-title { color: #f0f2ff; }

.sidebar-toggle {
  display: none;
  position: fixed;
  top: 64px;
  left: 16px;
  z-index: 200;
  width: 38px;
  height: 38px;
  background: var(--sidebar-bg);
  border: 1px solid #2e3248;
  border-radius: var(--radius);
  cursor: pointer;
  flex-direction: column;
  align-items: center;
  justify-content: center;
  gap: 4px;
}
.sidebar-toggle span {
  display: block;
  width: 18px;
  height: 2px;
  background: var(--sidebar-text);
  border-radius: 2px;
}
.sidebar-overlay {
  display: none;
  position: fixed;
  inset: 0;
  background: rgba(0, 0, 0, .5);
  z-index: 90;
}

.content {
  margin-left: var(--sidebar-w);
  flex: 1;
  min-height: 100vh;
  display: flex;
  flex-direction: column;
}
.lesson-page { display: flex; flex-direction: column; min-height: 100vh; }
.lesson-header {
  background: var(--bg-card);
  border-bottom: 1px solid var(--border);
  padding: 32px 40px 28px;
  position: relative;
}
.lesson-header::before {
  content: "";
  position: absolute;
  inset: 0 0 auto 0;
  height: 3px;
  background: var(--module-color, #888);
}
.lesson-breadcrumb {
  display: flex;
  align-items: center;
  flex-wrap: wrap;
  gap: 6px;
  font-size: 12px;
  color: var(--text-muted);
  margin-bottom: 12px;
}
.lesson-breadcrumb a:hover { color: var(--text); }
.lesson-breadcrumb-sep { color: var(--border); }
.lesson-header-row {
  display: flex;
  align-items: center;
  justify-content: space-between;
  flex-wrap: wrap;
  gap: 12px;
  margin-bottom: 10px;
}
.lesson-header-meta { display: flex; align-items: center; gap: 8px; }
.lesson-module-badge {
  font-size: 11px;
  font-weight: 700;
  letter-spacing: .06em;
  text-transform: uppercase;
  color: #fff;
  background: var(--module-color, #888);
  border-radius: 4px;
  padding: 3px 9px;
}
.lesson-id-badge {
  font-size: 11px;
  font-weight: 700;
  color: var(--text-muted);
  background: var(--bg);
  border: 1px solid var(--border);
  border-radius: 4px;
  padding: 3px 9px;
}
.view-toggle {
  display: flex;
  background: var(--bg);
  border: 1px solid var(--border);
  border-radius: 8px;
  padding: 3px;
  gap: 2px;
}
.view-toggle-btn {
  display: flex;
  align-items: center;
  gap: 6px;
  padding: 6px 12px;
  border: none;
  background: transparent;
  border-radius: 6px;
  font-size: 12px;
  font-weight: 600;
  color: var(--text-muted);
  cursor: pointer;
  transition: background var(--transition), color var(--transition);
  white-space: nowrap;
}
.view-toggle-btn:hover { background: var(--bg-card); color: var(--text); }
.view-toggle-btn.active {
  background: var(--bg-card);
  color: var(--module-color, var(--text));
  box-shadow: 0 1px 3px rgba(0, 0, 0, .08);
}
.lesson-header h1 {
  font-family: var(--font-heading);
  font-size: 32px;
  font-weight: 800;
  line-height: 1.22;
  color: var(--text);
  max-width: 820px;
}
.lesson-summary {
  max-width: 760px;
  color: var(--text-muted);
  font-size: 15px;
  margin-top: 10px;
}

.lesson-body {
  max-width: 860px;
  padding: 40px 40px 60px;
}
.lesson-body h2 {
  font-family: var(--font-heading);
  font-size: 26px;
  font-weight: 800;
  color: var(--text);
  margin: 40px 0 16px;
  padding-bottom: 10px;
  border-bottom: 2px solid var(--border);
}
.lesson-body h3 {
  font-family: var(--font-heading);
  font-size: 20px;
  font-weight: 700;
  color: var(--text);
  margin: 30px 0 12px;
}
.lesson-body section:first-child h2 { margin-top: 0; }
.lesson-body p {
  font-size: 17px;
  margin-bottom: 18px;
  color: #333;
  line-height: 1.8;
}
.lesson-body ul, .lesson-body ol {
  margin: 0 0 18px;
  padding-left: 26px;
}
.lesson-body li {
  font-size: 17px;
  margin-bottom: 8px;
  line-height: 1.75;
  color: #333;
}
.lesson-body li::marker { color: var(--module-color, #888); }
.lesson-body strong { font-weight: 700; color: var(--text); }
.lesson-body table {
  width: 100%;
  border-collapse: collapse;
  margin: 24px 0;
  font-size: 15px;
}
.lesson-body th {
  background: var(--bg);
  font-weight: 700;
  text-align: left;
  padding: 11px 16px;
  border-bottom: 2px solid var(--border);
  color: var(--text);
}
.lesson-body td {
  padding: 12px 16px;
  border-bottom: 1px solid var(--border);
  vertical-align: top;
}
.lesson-body code {
  font-family: "SF Mono", "Fira Code", monospace;
  font-size: 13px;
  background: #f0ede6;
  border: 1px solid var(--border);
  padding: 1px 6px;
  border-radius: 4px;
}
.original-note-hint {
  max-width: 620px;
  color: var(--text-muted) !important;
}
.original-note-list {
  display: grid;
  gap: 12px;
  margin-top: 22px;
}
.original-note-link {
  display: grid;
  grid-template-columns: auto 1fr auto;
  align-items: center;
  gap: 14px;
  padding: 16px;
  border: 1px solid var(--border);
  border-radius: 10px;
  background: var(--bg-card);
  color: var(--text);
  transition: border-color var(--transition), box-shadow var(--transition), transform var(--transition);
}
.original-note-link:hover {
  border-color: var(--module-color, #888);
  box-shadow: 0 10px 24px rgba(0, 0, 0, .08);
  transform: translateY(-1px);
}
.original-note-index {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  width: 28px;
  height: 28px;
  border-radius: 999px;
  background: var(--module-color-light, #eee);
  color: var(--module-color, #555);
  font-size: 13px;
  font-weight: 800;
}
.original-note-meta {
  display: grid;
  gap: 3px;
}
.original-note-meta strong {
  font-size: 16px;
}
.original-note-meta span {
  font-size: 13px;
  color: var(--text-muted);
}
.original-note-arrow {
  color: var(--module-color, #555);
  font-weight: 800;
}
.ipa {
  color: var(--text-muted);
  font-family: "SF Mono", "Fira Code", monospace;
  font-size: 13px;
}
.speak-btn {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  margin-left: 7px;
  padding: 3px 5px;
  background: transparent;
  border: 1px solid var(--border);
  border-radius: 5px;
  color: var(--text-light);
  cursor: pointer;
  vertical-align: middle;
  line-height: 1;
  transition: background var(--transition), color var(--transition), border-color var(--transition), transform 80ms ease;
}
.speak-btn:hover {
  background: var(--module-color-light, #f0f0f0);
  color: var(--module-color, #555);
  border-color: var(--module-color, #aaa);
}
.speak-btn:active { transform: scale(.92); }
.speak-btn.speaking {
  color: var(--module-color, #555);
  border-color: var(--module-color, #aaa);
  animation: pulse-speak .6s ease infinite alternate;
}

.selection-popup {
  position: fixed;
  z-index: 2000;
  width: 280px;
  display: none;
  background: #1e2130;
  border: 1px solid rgba(255, 255, 255, .12);
  border-radius: 12px;
  box-shadow: 0 8px 32px rgba(0, 0, 0, .35);
  padding: 12px 14px;
  color: #d8daf0;
}
.selection-popup.visible { display: block; animation: fadeIn .15s ease forwards; }
.selection-popup-header {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 8px;
  margin-bottom: 8px;
}
.selection-popup-label {
  color: #9aa0c0;
  font-size: 11px;
  font-weight: 700;
  letter-spacing: .06em;
  text-transform: uppercase;
}
.selection-popup-close {
  border: 0;
  background: transparent;
  color: #9aa0c0;
  cursor: pointer;
  font-size: 16px;
}
.selection-popup p {
  color: #b8bcd0;
  font-size: 13px;
  line-height: 1.5;
  margin-bottom: 10px;
}
.selection-speak-btn {
  gap: 6px;
  margin-left: 0;
  color: #d8daf0;
  background: rgba(255, 255, 255, .08);
  border-color: rgba(255, 255, 255, .14);
}
@keyframes pulse-speak {
  from { opacity: 1; }
  to { opacity: .45; }
}

.lesson-empty {
  min-height: 420px;
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
  text-align: center;
  padding: 80px 40px;
  color: var(--text-muted);
}
.lesson-empty-icon { font-size: 40px; margin-bottom: 16px; opacity: .4; }
.lesson-empty h2 {
  font-family: var(--font-heading);
  font-size: 22px;
  color: var(--text);
  margin-bottom: 8px;
  opacity: .7;
}
.lesson-empty p { font-size: 14px; max-width: 360px; }

.lesson-nav {
  margin-top: auto;
  border-top: 1px solid var(--border);
  display: flex;
  background: var(--bg-card);
}
.lesson-nav-btn {
  flex: 1;
  display: flex;
  align-items: center;
  padding: 20px 28px;
  gap: 12px;
  cursor: pointer;
  transition: background var(--transition);
  border: none;
  background: transparent;
  text-align: left;
  color: var(--text);
}
.lesson-nav-btn:hover { background: var(--bg); }
.lesson-nav-btn:disabled { opacity: .3; cursor: default; }
.lesson-nav-btn:disabled:hover { background: transparent; }
.lesson-nav-btn + .lesson-nav-btn {
  border-left: 1px solid var(--border);
  justify-content: flex-end;
  text-align: right;
}
.nav-btn-label {
  font-size: 10px;
  font-weight: 700;
  letter-spacing: .08em;
  text-transform: uppercase;
  color: var(--text-muted);
  display: block;
  margin-bottom: 2px;
}
.nav-btn-title {
  font-size: 14px;
  font-weight: 600;
  line-height: 1.3;
}
.nav-btn-arrow { font-size: 20px; color: var(--text-muted); flex-shrink: 0; }

@keyframes fadeIn {
  from { opacity: 0; transform: translateY(8px); }
  to { opacity: 1; transform: translateY(0); }
}
.fade-in { animation: fadeIn .25s ease forwards; }
::-webkit-scrollbar { width: 6px; height: 6px; }
::-webkit-scrollbar-track { background: transparent; }
::-webkit-scrollbar-thumb { background: #d0cdc6; border-radius: 3px; }

@media (max-width: 768px) {
  .sidebar { transform: translateX(-100%); }
  .sidebar.open { transform: translateX(0); box-shadow: var(--shadow-md); }
  .sidebar-overlay.visible { display: block; }
  .sidebar-toggle { display: flex; }
  .content { margin-left: 0; }
  .lesson-header { padding: 64px 20px 20px; }
  .lesson-header h1 { font-size: 29px; }
  .lesson-body { padding: 28px 20px 48px; }
  .lesson-body table { display: block; overflow-x: auto; }
  .lesson-nav-btn { padding: 16px 20px; }
}