/* ============================================================
   Algorithm Study Dashboard — Design System & Styles
   Inspired by Notion, Vercel, Stripe Docs
   ============================================================ */

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

:root {
  --color-bg: #FAFAFA;
  --color-surface: #FFFFFF;
  --color-surface-hover: #F5F5F5;
  --color-surface-active: #F0F0F0;
  --color-border: #E5E5E5;
  --color-border-light: #F0F0F0;
  --color-border-dark: #D4D4D4;

  --color-text-primary: #18181B;
  --color-text-secondary: #52525B;
  --color-text-tertiary: #A1A1AA;
  --color-text-muted: #D4D4D8;

  --color-accent: #18181B;
  --color-accent-light: #F4F4F5;
  --color-accent-hover: #27272A;
  --color-accent-subtle: #E4E4E7;

  --color-easy: #16A34A;
  --color-easy-bg: #F0FDF4;
  --color-easy-border: #BBF7D0;
  --color-medium: #D97706;
  --color-medium-bg: #FFFBEB;
  --color-medium-border: #FDE68A;
  --color-hard: #DC2626;
  --color-hard-bg: #FEF2F2;
  --color-hard-border: #FECACA;

  --shadow-xs: 0 1px 2px rgba(0,0,0,0.04);
  --shadow-sm: 0 1px 3px rgba(0,0,0,0.06), 0 1px 2px rgba(0,0,0,0.04);
  --shadow-md: 0 4px 6px -1px rgba(0,0,0,0.06), 0 2px 4px -2px rgba(0,0,0,0.04);
  --shadow-lg: 0 10px 15px -3px rgba(0,0,0,0.06), 0 4px 6px -4px rgba(0,0,0,0.04);

  --sidebar-width: 280px;
  --font-sans: 'Inter', -apple-system, BlinkMacSystemFont, 'Segoe UI', sans-serif;
  --font-mono: 'JetBrains Mono', 'SF Mono', 'Fira Code', monospace;

  --radius-sm: 6px;
  --radius-md: 8px;
  --radius-lg: 12px;
  --radius-xl: 16px;

  --transition-fast: 150ms cubic-bezier(0.4, 0, 0.2, 1);
  --transition-base: 200ms cubic-bezier(0.4, 0, 0.2, 1);
  --transition-slow: 300ms cubic-bezier(0.4, 0, 0.2, 1);
}

html { font-size: 16px; -webkit-font-smoothing: antialiased; -moz-osx-font-smoothing: grayscale; }

body {
  font-family: var(--font-sans);
  background: var(--color-bg);
  color: var(--color-text-primary);
  line-height: 1.6;
  overflow: hidden;
  height: 100vh;
}

/* ── App Layout ───────────────────────────────────────────── */
#app { display: flex; height: 100vh; overflow: hidden; }

/* ── Sidebar ──────────────────────────────────────────────── */
.sidebar {
  width: var(--sidebar-width);
  min-width: var(--sidebar-width);
  height: 100vh;
  background: var(--color-surface);
  border-right: 1px solid var(--color-border);
  display: flex;
  flex-direction: column;
  transition: transform var(--transition-slow), min-width var(--transition-slow), width var(--transition-slow);
  z-index: 100;
  overflow: hidden;
}

.sidebar.collapsed { min-width: 0; width: 0; transform: translateX(-100%); }

/* Sidebar reopen button — only visible when sidebar is collapsed */
.sidebar-reopen {
  display: none; position: fixed; top: 14px; left: 14px; z-index: 200;
  width: 36px; height: 36px; border: 1px solid var(--color-border);
  background: var(--color-surface); border-radius: var(--radius-md);
  align-items: center; justify-content: center; cursor: pointer;
  color: var(--color-text-tertiary); box-shadow: var(--shadow-sm);
  transition: all var(--transition-fast);
}
.sidebar-reopen:hover { background: var(--color-surface-hover); color: var(--color-text-primary); box-shadow: var(--shadow-md); }
.sidebar.collapsed ~ .main-content .sidebar-reopen { display: flex; }

.sidebar-header {
  display: flex; align-items: center; justify-content: space-between;
  padding: 16px 20px; height: 56px;
  border-bottom: 1px solid var(--color-border-light); flex-shrink: 0;
}

.logo { display: flex; align-items: center; gap: 10px; }
.logo-text { font-size: 15px; font-weight: 700; letter-spacing: -0.3px; }

.sidebar-toggle {
  display: flex; align-items: center; justify-content: center;
  width: 32px; height: 32px; border: none; background: transparent;
  border-radius: var(--radius-sm); color: var(--color-text-tertiary);
  cursor: pointer; transition: background var(--transition-fast), color var(--transition-fast);
}
.sidebar-toggle:hover { background: var(--color-surface-hover); color: var(--color-text-secondary); }

.sidebar-search {
  padding: 12px 16px; border-bottom: 1px solid var(--color-border-light);
  display: flex; align-items: center; gap: 8px;
  color: var(--color-text-tertiary); flex-shrink: 0;
}
.sidebar-search input {
  border: none; background: none; outline: none;
  font-family: var(--font-sans); font-size: 13px;
  color: var(--color-text-primary); width: 100%; line-height: 1.5;
}
.sidebar-search input::placeholder { color: var(--color-text-tertiary); }

.sidebar-nav {
  flex: 1; overflow-y: auto; padding: 8px 0;
  scrollbar-width: thin; scrollbar-color: var(--color-border) transparent;
}
.sidebar-nav::-webkit-scrollbar { width: 4px; }
.sidebar-nav::-webkit-scrollbar-track { background: transparent; }
.sidebar-nav::-webkit-scrollbar-thumb { background: var(--color-border); border-radius: 4px; }

.nav-topic { margin: 2px 8px; }

.nav-topic-btn {
  display: flex; align-items: center; gap: 8px;
  width: 100%; padding: 8px 12px; border: none; background: none;
  font-family: var(--font-sans); font-size: 13px; font-weight: 600;
  color: var(--color-text-secondary); cursor: pointer;
  border-radius: var(--radius-sm); transition: background var(--transition-fast), color var(--transition-fast);
  text-align: left; line-height: 1.4;
}
.nav-topic-btn:hover { background: var(--color-surface-hover); color: var(--color-text-primary); }
.nav-topic-btn.active { background: var(--color-surface-active); color: var(--color-text-primary); }

.nav-topic-icon { font-size: 16px; line-height: 1; flex-shrink: 0; width: 20px; text-align: center; }

.nav-topic-chevron {
  margin-left: auto; display: flex;
  transition: transform var(--transition-fast); color: var(--color-text-tertiary); flex-shrink: 0;
}
.nav-topic-btn.expanded .nav-topic-chevron { transform: rotate(90deg); }

.nav-methods { overflow: hidden; max-height: 0; transition: max-height var(--transition-slow); }
.nav-methods.expanded { max-height: 600px; }

.nav-method-btn {
  display: flex; align-items: center; width: calc(100% - 8px); margin-left: 8px;
  padding: 6px 12px 6px 32px; border: none; background: none;
  font-family: var(--font-sans); font-size: 13px; font-weight: 400;
  color: var(--color-text-tertiary); cursor: pointer;
  border-radius: var(--radius-sm); transition: all var(--transition-fast);
  text-align: left; line-height: 1.5; position: relative;
}
.nav-method-btn:hover { color: var(--color-text-secondary); background: var(--color-surface-hover); }
.nav-method-btn.active {
  color: var(--color-text-primary); background: var(--color-accent-light); font-weight: 500;
}
.nav-method-btn.active::before {
  content: ''; position: absolute; left: 0; top: 4px; bottom: 4px;
  width: 2px; background: var(--color-accent); border-radius: 2px;
}

.nav-method-count {
  margin-left: auto; font-size: 11px; font-weight: 500;
  color: var(--color-text-muted); background: var(--color-border-light);
  padding: 1px 6px; border-radius: 10px; line-height: 1.5;
}

/* Sidebar Actions */
.sidebar-actions {
  padding: 8px; border-top: 1px solid var(--color-border-light); flex-shrink: 0;
  display: flex; flex-direction: column; gap: 2px;
}

.sidebar-action-btn {
  display: flex; align-items: center; gap: 8px;
  padding: 8px 12px; border: none; background: none;
  font-family: var(--font-sans); font-size: 13px; font-weight: 500;
  color: var(--color-text-tertiary); cursor: pointer;
  border-radius: var(--radius-sm); transition: all var(--transition-fast);
  width: 100%; text-align: left;
}
.sidebar-action-btn:hover { background: var(--color-surface-hover); color: var(--color-text-secondary); }

/* Sidebar Footer */
.sidebar-footer { padding: 16px; border-top: 1px solid var(--color-border-light); flex-shrink: 0; }
.progress-label {
  display: flex; justify-content: space-between; font-size: 12px;
  color: var(--color-text-tertiary); margin-bottom: 8px; font-weight: 500;
}
.progress-bar { height: 4px; background: var(--color-border-light); border-radius: 4px; overflow: hidden; }
.progress-fill {
  height: 100%; background: var(--color-text-primary);
  border-radius: 4px; transition: width var(--transition-slow);
}

/* ── Main Content ─────────────────────────────────────────── */
.main-content { flex: 1; overflow-y: auto; overflow-x: hidden; scroll-behavior: smooth; }
.main-content::-webkit-scrollbar { width: 6px; }
.main-content::-webkit-scrollbar-track { background: transparent; }
.main-content::-webkit-scrollbar-thumb { background: var(--color-border); border-radius: 6px; }
.main-content { scrollbar-width: thin; scrollbar-color: var(--color-border) transparent; }

/* ── Views ────────────────────────────────────────────────── */
.view-panel { display: none; animation: fadeIn var(--transition-slow) ease; }
.view-panel.active { display: block; }
@keyframes fadeIn { from { opacity: 0; transform: translateY(8px); } to { opacity: 1; transform: translateY(0); } }

.loading-view.active {
  display: flex; align-items: center; justify-content: center;
  height: 100%; color: var(--color-text-tertiary);
  font-size: 14px; font-weight: 500;
}

/* ── Welcome View ─────────────────────────────────────────── */
.welcome-inner { max-width: 720px; margin: 0 auto; padding: 80px 40px; }

.welcome-badge {
  display: inline-block; font-size: 12px; font-weight: 600;
  letter-spacing: 0.5px; text-transform: uppercase;
  color: var(--color-text-tertiary); background: var(--color-surface);
  border: 1px solid var(--color-border); padding: 4px 12px;
  border-radius: 20px; margin-bottom: 24px;
}

.welcome-title {
  font-size: 42px; font-weight: 700; letter-spacing: -1.2px;
  line-height: 1.15; color: var(--color-text-primary); margin-bottom: 16px;
}

.welcome-subtitle {
  font-size: 17px; line-height: 1.7; color: var(--color-text-secondary);
  max-width: 520px; margin-bottom: 48px;
}

.welcome-stats { display: grid; grid-template-columns: repeat(4, 1fr); gap: 12px; margin-bottom: 32px; }

.stat-card {
  background: var(--color-surface); border: 1px solid var(--color-border);
  border-radius: var(--radius-lg); padding: 20px;
  transition: box-shadow var(--transition-base), border-color var(--transition-base);
}
.stat-card:hover { box-shadow: var(--shadow-md); border-color: var(--color-border-dark); }
.stat-card.clickable { cursor: pointer; }

.stat-number {
  font-size: 26px; font-weight: 700; letter-spacing: -0.5px;
  color: var(--color-text-primary); margin-bottom: 4px;
}
.stat-sub { font-size: 16px; font-weight: 500; color: var(--color-text-tertiary); }
.stat-label { font-size: 12px; color: var(--color-text-tertiary); font-weight: 500; }

.welcome-cards { display: grid; grid-template-columns: repeat(2, 1fr); gap: 12px; margin-top: 16px; }

.welcome-card {
  background: var(--color-surface); border: 1px solid var(--color-border);
  border-radius: var(--radius-lg); padding: 20px 24px; cursor: pointer;
  transition: all var(--transition-base); display: flex; align-items: center; gap: 14px;
}
.welcome-card:hover { border-color: var(--color-border-dark); box-shadow: var(--shadow-md); transform: translateY(-1px); }

.welcome-card-icon {
  font-size: 24px; width: 44px; height: 44px;
  display: flex; align-items: center; justify-content: center;
  background: var(--color-bg); border-radius: var(--radius-md); flex-shrink: 0;
}
.welcome-card-info h3 { font-size: 14px; font-weight: 600; margin-bottom: 2px; }
.welcome-card-info p { font-size: 12px; color: var(--color-text-tertiary); }

/* ── Content Header / Back Button ─────────────────────────── */
.content-header {
  padding: 16px 40px; border-bottom: 1px solid var(--color-border-light);
  position: sticky; top: 0;
  background: rgba(250, 250, 250, 0.85);
  backdrop-filter: blur(12px); -webkit-backdrop-filter: blur(12px); z-index: 10;
}

.back-btn {
  display: inline-flex; align-items: center; gap: 6px;
  border: none; background: none; font-family: var(--font-sans);
  font-size: 13px; font-weight: 500; color: var(--color-text-tertiary);
  cursor: pointer; padding: 4px 8px; margin: -4px -8px;
  border-radius: var(--radius-sm);
  transition: color var(--transition-fast), background var(--transition-fast);
}
.back-btn:hover { color: var(--color-text-primary); background: var(--color-surface-hover); }

/* ── Topic View ───────────────────────────────────────────── */
.topic-header {
  display: flex; align-items: center; gap: 20px;
  padding: 40px 40px 0; max-width: 880px;
}
.topic-icon {
  width: 56px; height: 56px; display: flex; align-items: center; justify-content: center;
  background: var(--color-surface); border: 1px solid var(--color-border);
  border-radius: var(--radius-lg); font-size: 28px; flex-shrink: 0;
}
.topic-title { font-size: 30px; font-weight: 700; letter-spacing: -0.8px; line-height: 1.2; }
.topic-desc { font-size: 15px; color: var(--color-text-secondary); margin-top: 4px; line-height: 1.5; }

.topic-actions {
  display: flex; align-items: center; gap: 12px; flex-wrap: wrap;
  padding: 24px 40px; max-width: 880px;
}
.topic-method-count { font-size: 13px; color: var(--color-text-tertiary); font-weight: 500; }

/* Revision header */
.revision-header {
  display: flex; align-items: center; gap: 20px;
  padding: 40px 40px 0; max-width: 880px;
}
.revision-icon {
  width: 56px; height: 56px; display: flex; align-items: center; justify-content: center;
  background: var(--color-medium-bg); border: 1px solid var(--color-medium-border);
  border-radius: var(--radius-lg); font-size: 28px; flex-shrink: 0;
}
.revision-filters { padding: 24px 40px 16px; }

/* ── Buttons ──────────────────────────────────────────────── */
.btn {
  display: inline-flex; align-items: center; gap: 8px;
  padding: 10px 20px; border: none; border-radius: var(--radius-md);
  font-family: var(--font-sans); font-size: 13px; font-weight: 600;
  cursor: pointer; transition: all var(--transition-fast); line-height: 1;
}
.btn-primary { background: var(--color-accent); color: #fff; }
.btn-primary:hover { background: var(--color-accent-hover); box-shadow: var(--shadow-sm); }
.btn-secondary {
  background: var(--color-surface); color: var(--color-text-primary);
  border: 1px solid var(--color-border);
}
.btn-secondary:hover { background: var(--color-surface-hover); border-color: var(--color-border-dark); }
.btn-ghost {
  background: none; color: var(--color-text-tertiary); border: none; padding: 8px 14px;
}
.btn-ghost:hover { color: var(--color-text-primary); background: var(--color-surface-hover); }
.btn-sm { padding: 6px 12px; font-size: 12px; }

.btn-icon {
  display: flex; align-items: center; justify-content: center;
  width: 32px; height: 32px; border: none; background: transparent;
  border-radius: var(--radius-sm); color: var(--color-text-tertiary);
  cursor: pointer; transition: all var(--transition-fast);
}
.btn-icon:hover { background: var(--color-surface-hover); color: var(--color-text-primary); }

.btn-icon-sm {
  display: inline-flex; align-items: center; justify-content: center;
  width: 24px; height: 24px; border: none; background: transparent;
  border-radius: 4px; color: var(--color-text-muted);
  cursor: pointer; transition: all var(--transition-fast);
}
.btn-icon-sm:hover { background: var(--color-surface-active); color: var(--color-text-secondary); }

.btn-text-sm {
  border: none; background: none; color: var(--color-text-muted);
  cursor: pointer; font-size: 14px; padding: 0 4px; font-weight: 600;
  transition: color var(--transition-fast);
}
.btn-text-sm:hover { color: var(--color-hard); }

/* ── All Questions Panel ──────────────────────────────────── */
.all-questions-panel {
  margin: 0 40px 32px; max-width: 800px;
  max-height: 0; overflow: hidden; opacity: 0;
  transition: max-height var(--transition-slow), opacity var(--transition-base), margin var(--transition-base);
}
.all-questions-panel.visible { max-height: 5000px; opacity: 1; margin-bottom: 32px; }

.all-questions-header {
  display: flex; align-items: center; justify-content: space-between;
  margin-bottom: 16px; flex-wrap: wrap; gap: 12px;
}
.all-questions-header h2 { font-size: 18px; font-weight: 600; letter-spacing: -0.3px; }

.filter-group {
  display: flex; gap: 4px;
  background: var(--color-surface); border: 1px solid var(--color-border);
  border-radius: var(--radius-md); padding: 3px;
}
.filter-btn {
  padding: 5px 14px; border: none; background: none;
  font-family: var(--font-sans); font-size: 12px; font-weight: 500;
  color: var(--color-text-tertiary); cursor: pointer;
  border-radius: var(--radius-sm); transition: all var(--transition-fast); line-height: 1.4;
}
.filter-btn:hover { color: var(--color-text-secondary); }
.filter-btn.active { background: var(--color-accent); color: #fff; }

/* ── Method Cards (Topic View) ────────────────────────────── */
.method-cards { padding: 0 40px 60px; max-width: 880px; display: grid; grid-template-columns: 1fr; gap: 12px; }

.method-card {
  background: var(--color-surface); border: 1px solid var(--color-border);
  border-radius: var(--radius-lg); padding: 24px; cursor: pointer;
  transition: all var(--transition-base);
  display: flex; align-items: flex-start; justify-content: space-between; gap: 16px;
}
.method-card:hover { border-color: var(--color-border-dark); box-shadow: var(--shadow-md); transform: translateY(-1px); }
.method-card-info h3 { font-size: 15px; font-weight: 600; margin-bottom: 4px; letter-spacing: -0.2px; }
.method-card-info p {
  font-size: 13px; color: var(--color-text-tertiary); line-height: 1.5;
  display: -webkit-box; -webkit-line-clamp: 2; -webkit-box-orient: vertical; overflow: hidden;
}
.method-card-meta { display: flex; align-items: center; gap: 8px; flex-shrink: 0; }
.method-card-count {
  font-size: 12px; font-weight: 500; color: var(--color-text-tertiary);
  background: var(--color-bg); padding: 4px 10px; border-radius: 20px; white-space: nowrap;
}
.method-card-solved {
  font-size: 11px; font-weight: 600; color: var(--color-easy);
  background: var(--color-easy-bg); border: 1px solid var(--color-easy-border);
  padding: 3px 8px; border-radius: 20px; white-space: nowrap;
}
.method-card-arrow {
  color: var(--color-text-muted);
  transition: transform var(--transition-fast), color var(--transition-fast);
}
.method-card:hover .method-card-arrow { transform: translateX(2px); color: var(--color-text-secondary); }

/* ── Method Detail View ───────────────────────────────────── */
.method-header { padding: 40px 40px 0; max-width: 760px; }
.method-badge {
  display: inline-block; font-size: 11px; font-weight: 600;
  letter-spacing: 0.5px; text-transform: uppercase;
  color: var(--color-text-tertiary); background: var(--color-bg);
  border: 1px solid var(--color-border); padding: 3px 10px;
  border-radius: 20px; margin-bottom: 12px;
}
.method-title { font-size: 28px; font-weight: 700; letter-spacing: -0.7px; line-height: 1.25; }

.method-explanation { padding: 24px 40px 0; max-width: 760px; font-size: 15px; line-height: 1.8; color: var(--color-text-secondary); }
.method-explanation p { margin-bottom: 16px; }
.method-explanation strong { color: var(--color-text-primary); font-weight: 600; }
.method-explanation code {
  font-family: var(--font-mono); font-size: 13px;
  background: var(--color-bg); border: 1px solid var(--color-border-light);
  padding: 2px 6px; border-radius: 4px; color: var(--color-text-primary);
}
.method-explanation ul, .method-explanation ol { padding-left: 20px; margin-bottom: 16px; }
.method-explanation li { margin-bottom: 6px; }
.method-explanation h3 { font-size: 16px; font-weight: 600; margin: 28px 0 12px; letter-spacing: -0.2px; }

.explanation-section { margin-bottom: 20px; }
.explanation-callout {
  background: var(--color-bg); border: 1px solid var(--color-border);
  border-radius: var(--radius-md); padding: 16px 20px; margin: 16px 0; font-size: 14px;
}
.explanation-callout strong { display: block; margin-bottom: 4px; }

.complexity-badges { display: flex; gap: 12px; margin-top: 12px; flex-wrap: wrap; }
.complexity-badge {
  display: inline-flex; align-items: center; gap: 6px;
  font-size: 13px; font-weight: 500; color: var(--color-text-secondary);
  background: var(--color-surface); border: 1px solid var(--color-border);
  padding: 6px 14px; border-radius: var(--radius-sm);
}
.complexity-badge code {
  font-family: var(--font-mono); font-size: 12px;
  color: var(--color-text-primary); font-weight: 600;
}

.method-question-actions {
  display: flex; gap: 8px; padding: 32px 40px 0; align-items: center; flex-wrap: wrap;
}
.show-questions-btn .chevron-icon { transition: transform var(--transition-fast); }
.show-questions-btn.expanded .chevron-icon { transform: rotate(180deg); }

.method-questions {
  max-height: 0; overflow: hidden; opacity: 0;
  transition: max-height 400ms cubic-bezier(0.4, 0, 0.2, 1), opacity var(--transition-base);
  padding: 0 40px;
}
.method-questions.visible { max-height: 5000px; opacity: 1; padding-top: 24px; padding-bottom: 60px; }
.method-questions-inner { max-width: 780px; }

/* ── Questions ────────────────────────────────────────────── */
.questions-grid { display: flex; flex-direction: column; gap: 6px; }

.question-item {
  display: flex; align-items: center; gap: 10px;
  padding: 12px 16px; background: var(--color-surface);
  border: 1px solid var(--color-border); border-radius: var(--radius-md);
  transition: all var(--transition-base); animation: slideUp var(--transition-slow) ease both;
}
.question-item.checked { background: var(--color-accent-light); border-color: var(--color-accent-subtle); }
.question-item:hover { border-color: var(--color-border-dark); box-shadow: var(--shadow-xs); }

@keyframes slideUp { from { opacity: 0; transform: translateY(8px); } to { opacity: 1; transform: translateY(0); } }

.question-check {
  border: none; background: none; cursor: pointer; padding: 0;
  display: flex; align-items: center; flex-shrink: 0;
  transition: transform var(--transition-fast);
}
.question-check:hover { transform: scale(1.1); }

.question-number {
  font-size: 11px; font-weight: 600; color: var(--color-text-muted);
  min-width: 20px; font-family: var(--font-mono);
}

.question-info { flex: 1; min-width: 0; display: flex; flex-direction: column; gap: 2px; }
.question-title { font-size: 13px; font-weight: 500; color: var(--color-text-primary); }
.question-title.solved { text-decoration: line-through; color: var(--color-text-tertiary); }
.question-remark {
  font-size: 11px; color: var(--color-text-tertiary); font-style: italic;
  white-space: nowrap; overflow: hidden; text-overflow: ellipsis;
}
.question-note-indicator {
  font-size: 10px; color: var(--color-medium); font-weight: 600;
}

.question-method-tag {
  font-size: 11px; font-weight: 500; color: var(--color-text-tertiary);
  background: var(--color-bg); padding: 2px 8px; border-radius: 4px; flex-shrink: 0;
  max-width: 120px; white-space: nowrap; overflow: hidden; text-overflow: ellipsis;
}

.difficulty-tag {
  font-size: 10px; font-weight: 600; padding: 3px 8px;
  border-radius: 20px; text-transform: uppercase; letter-spacing: 0.3px; flex-shrink: 0;
}
.difficulty-tag.easy { color: var(--color-easy); background: var(--color-easy-bg); border: 1px solid var(--color-easy-border); }
.difficulty-tag.medium { color: var(--color-medium); background: var(--color-medium-bg); border: 1px solid var(--color-medium-border); }
.difficulty-tag.hard { color: var(--color-hard); background: var(--color-hard-bg); border: 1px solid var(--color-hard-border); }

.question-action-btn {
  display: flex; align-items: center; justify-content: center;
  width: 28px; height: 28px; border: none; background: transparent;
  border-radius: var(--radius-sm); color: var(--color-text-muted);
  cursor: pointer; transition: all var(--transition-fast); flex-shrink: 0;
}
.question-action-btn:hover { background: var(--color-surface-hover); color: var(--color-text-secondary); }
.question-action-btn.revision-btn.active { color: var(--color-medium); }

.question-link {
  display: inline-flex; align-items: center; gap: 4px;
  font-size: 12px; font-weight: 500; color: var(--color-text-tertiary);
  text-decoration: none; padding: 4px 10px;
  border-radius: var(--radius-sm); transition: all var(--transition-fast); flex-shrink: 0;
}
.question-link:hover { color: var(--color-text-primary); background: var(--color-surface-hover); }

.difficulty-section { margin-bottom: 24px; }
.difficulty-section-header {
  font-size: 12px; font-weight: 600; text-transform: uppercase; letter-spacing: 0.5px;
  color: var(--color-text-tertiary); margin-bottom: 10px; padding-bottom: 8px;
  border-bottom: 1px solid var(--color-border-light);
  display: flex; align-items: center; gap: 8px;
}
.difficulty-dot { width: 8px; height: 8px; border-radius: 50%; }
.difficulty-dot.easy { background: var(--color-easy); }
.difficulty-dot.medium { background: var(--color-medium); }
.difficulty-dot.hard { background: var(--color-hard); }

/* ── Notes ────────────────────────────────────────────────── */
.notes-section { margin-bottom: 24px; }
.notes-header {
  display: flex; align-items: center; justify-content: space-between;
  margin-bottom: 8px;
}
.notes-header h3 {
  font-size: 13px; font-weight: 600; color: var(--color-text-tertiary);
  display: flex; align-items: center; gap: 6px;
}

.notes-list { display: flex; flex-direction: column; gap: 6px; }

.note-card {
  background: var(--color-surface); border: 1px solid var(--color-border);
  border-radius: var(--radius-md); padding: 12px 16px;
  transition: border-color var(--transition-fast);
}
.note-card:hover { border-color: var(--color-border-dark); }
.note-content { font-size: 13px; color: var(--color-text-secondary); line-height: 1.6; white-space: pre-wrap; }
.note-meta {
  display: flex; align-items: center; justify-content: space-between;
  margin-top: 8px; font-size: 11px; color: var(--color-text-muted);
}
.note-actions { display: flex; gap: 4px; }

.notes-list-compact { display: flex; flex-direction: column; gap: 4px; margin-bottom: 8px; }
.note-compact {
  font-size: 13px; color: var(--color-text-secondary); padding: 8px 12px;
  background: var(--color-bg); border-radius: var(--radius-sm);
  display: flex; align-items: center; justify-content: space-between; gap: 8px;
}

/* ── AI Chat Panel ────────────────────────────────────────── */
.ai-fab {
  position: fixed; bottom: 28px; right: 28px; z-index: 300;
  width: 52px; height: 52px;
  border: 1px solid var(--color-border); background: var(--color-surface);
  border-radius: 50%; display: flex; align-items: center; justify-content: center;
  cursor: pointer; color: var(--color-text-secondary);
  box-shadow: var(--shadow-lg);
  transition: all var(--transition-base);
}
.ai-fab:hover { box-shadow: var(--shadow-lg); border-color: var(--color-border-dark); color: var(--color-text-primary); transform: scale(1.05); }
.ai-fab.active { background: var(--color-accent); color: #fff; border-color: var(--color-accent); }

.ai-panel {
  position: fixed; bottom: 92px; right: 28px; z-index: 300;
  width: 400px; max-height: 560px;
  background: var(--color-surface); border: 1px solid var(--color-border);
  border-radius: var(--radius-xl); box-shadow: var(--shadow-lg);
  display: flex; flex-direction: column;
  opacity: 0; transform: translateY(12px) scale(0.95);
  pointer-events: none;
  transition: opacity var(--transition-base), transform var(--transition-base);
}
.ai-panel.open { opacity: 1; transform: translateY(0) scale(1); pointer-events: all; }

.ai-panel-header {
  display: flex; align-items: center; justify-content: space-between;
  padding: 16px 20px; border-bottom: 1px solid var(--color-border-light); flex-shrink: 0;
}
.ai-panel-title {
  display: flex; align-items: center; gap: 8px;
  font-size: 14px; font-weight: 600; color: var(--color-text-primary);
}
.ai-panel-actions { display: flex; gap: 4px; align-items: center; }

.ai-model-select {
  padding: 4px 8px; border: 1px solid var(--color-border);
  border-radius: var(--radius-sm); font-family: var(--font-sans); font-size: 11px;
  background: var(--color-bg); color: var(--color-text-secondary); outline: none;
  cursor: pointer; transition: border-color var(--transition-fast);
}
.ai-model-select:focus { border-color: var(--color-accent); }

.ai-messages {
  flex: 1; overflow-y: auto; padding: 16px 20px;
  display: flex; flex-direction: column; gap: 12px;
  min-height: 200px; max-height: 360px;
  scrollbar-width: thin; scrollbar-color: var(--color-border) transparent;
}

.ai-welcome-msg {
  font-size: 13px; color: var(--color-text-secondary); line-height: 1.6;
}
.ai-welcome-msg p { margin-bottom: 8px; }
.ai-welcome-msg ul { padding-left: 16px; margin: 0; }
.ai-welcome-msg li { font-size: 12px; color: var(--color-text-tertiary); margin-bottom: 4px; }

.ai-msg { font-size: 13px; line-height: 1.5; animation: fadeIn var(--transition-base) ease; }
.ai-msg-user {
  align-self: flex-end; background: var(--color-accent); color: #fff;
  padding: 8px 14px; border-radius: 12px 12px 2px 12px; max-width: 85%;
}
.ai-msg-system {
  align-self: flex-start;
}
.ai-system-msg {
  font-size: 13px; color: var(--color-text-secondary);
  background: var(--color-bg); padding: 8px 14px;
  border-radius: 12px 12px 12px 2px; display: inline-block; max-width: 85%;
}

.ai-input-area {
  display: flex; gap: 8px; padding: 16px 20px;
  border-top: 1px solid var(--color-border-light); flex-shrink: 0;
}
.ai-input-area textarea {
  flex: 1; border: 1px solid var(--color-border); border-radius: var(--radius-md);
  padding: 8px 12px; font-family: var(--font-sans); font-size: 13px;
  color: var(--color-text-primary); resize: none; outline: none;
  background: var(--color-bg);
  transition: border-color var(--transition-fast);
}
.ai-input-area textarea:focus { border-color: var(--color-accent); }

.ai-send-btn {
  display: flex; align-items: center; justify-content: center;
  width: 36px; height: 36px; border: none;
  background: var(--color-accent); color: #fff;
  border-radius: var(--radius-md); cursor: pointer;
  transition: background var(--transition-fast); flex-shrink: 0; align-self: flex-end;
}
.ai-send-btn:hover { background: var(--color-accent-hover); }

/* ── Modal ────────────────────────────────────────────────── */
.modal-overlay {
  position: fixed; inset: 0; z-index: 500;
  background: rgba(0, 0, 0, 0.3);
  backdrop-filter: blur(4px); -webkit-backdrop-filter: blur(4px);
  display: flex; align-items: center; justify-content: center;
  opacity: 0; pointer-events: none;
  transition: opacity var(--transition-base);
}
.modal-overlay.visible { opacity: 1; pointer-events: all; }

.modal {
  background: var(--color-surface); border-radius: var(--radius-xl);
  box-shadow: var(--shadow-lg); width: 440px; max-width: 90vw;
  max-height: 80vh; overflow-y: auto;
  transform: translateY(12px) scale(0.97);
  transition: transform var(--transition-base);
}
.modal-overlay.visible .modal { transform: translateY(0) scale(1); }

.modal-header {
  display: flex; align-items: center; justify-content: space-between;
  padding: 20px 24px 0;
}
.modal-header h2 { font-size: 16px; font-weight: 600; letter-spacing: -0.2px; }

.modal-body { padding: 20px 24px; }

.modal-footer {
  display: flex; gap: 8px; justify-content: flex-end;
  padding: 0 24px 20px;
}

.modal-label {
  display: block; font-size: 12px; font-weight: 600;
  color: var(--color-text-tertiary); margin-bottom: 6px; margin-top: 12px;
}
.modal-label:first-child { margin-top: 0; }

.modal-input {
  width: 100%; padding: 10px 14px; border: 1px solid var(--color-border);
  border-radius: var(--radius-md); font-family: var(--font-sans); font-size: 13px;
  color: var(--color-text-primary); outline: none; background: var(--color-bg);
  transition: border-color var(--transition-fast);
}
.modal-input:focus { border-color: var(--color-accent); }

.modal-textarea {
  width: 100%; padding: 10px 14px; border: 1px solid var(--color-border);
  border-radius: var(--radius-md); font-family: var(--font-sans); font-size: 13px;
  color: var(--color-text-primary); outline: none; background: var(--color-bg);
  resize: vertical; transition: border-color var(--transition-fast);
}
.modal-textarea:focus { border-color: var(--color-accent); }

.modal-select {
  width: 100%; padding: 10px 14px; border: 1px solid var(--color-border);
  border-radius: var(--radius-md); font-family: var(--font-sans); font-size: 13px;
  color: var(--color-text-primary); outline: none; background: var(--color-bg);
  cursor: pointer;
}

.modal-divider { height: 1px; background: var(--color-border-light); margin: 16px 0; }
.modal-hint { font-size: 12px; color: var(--color-text-tertiary); margin-bottom: 8px; }

/* ── Empty State ──────────────────────────────────────────── */
.empty-state { text-align: center; padding: 48px 24px; color: var(--color-text-tertiary); }
.empty-state p { font-size: 14px; font-weight: 500; }

/* ── Responsive ───────────────────────────────────────────── */
@media (max-width: 768px) {
  :root { --sidebar-width: 260px; }
  .welcome-inner { padding: 48px 24px; }
  .welcome-title { font-size: 30px; }
  .welcome-stats { grid-template-columns: repeat(2, 1fr); }
  .welcome-cards { grid-template-columns: 1fr; }
  .content-header, .topic-header, .topic-actions, .method-cards,
  .method-header, .method-explanation, .method-question-actions,
  .method-questions, .all-questions-panel, .revision-header, .revision-filters,
  #topic-notes-section, #method-notes-section { padding-left: 24px; padding-right: 24px; }
  .sidebar { position: fixed; left: 0; top: 0; }
  .ai-panel { width: calc(100vw - 32px); right: 16px; bottom: 80px; }
  .ai-fab { bottom: 20px; right: 20px; }
}

@media (max-width: 480px) {
  .welcome-stats { grid-template-columns: 1fr; }
  .question-item { flex-wrap: wrap; }
  .topic-actions { flex-direction: column; align-items: flex-start; }
}

/* ── Question Note Cards (inside modal) ───────────────────── */
.qnote-section { }

.qnote-card {
  background: var(--color-background);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-md);
  transition: border-color var(--transition-fast), box-shadow var(--transition-fast);
}
.qnote-card:hover { border-color: var(--color-border-hover); box-shadow: var(--shadow-sm); }

.qnote-card-body { padding: 12px 14px; }

.qnote-card-text {
  font-size: 13px; line-height: 1.6;
  color: var(--color-text-primary);
  white-space: pre-wrap; word-break: break-word;
}

.qnote-card-meta {
  display: flex; align-items: center; justify-content: space-between;
  margin-top: 8px; padding-top: 8px;
  border-top: 1px solid var(--color-border);
  font-size: 11px; color: var(--color-text-tertiary);
}

.qnote-card-actions { display: flex; gap: 2px; }

.qnote-action-btn {
  display: inline-flex; align-items: center; justify-content: center;
  width: 26px; height: 26px; border: none; background: none;
  border-radius: var(--radius-sm); cursor: pointer;
  color: var(--color-text-tertiary); transition: all var(--transition-fast);
}
.qnote-action-btn:hover { background: var(--color-surface-hover); color: var(--color-text-primary); }
.qnote-delete-btn:hover { color: var(--color-hard); background: #FEF2F2; }