/* ==========================================================================
   CSS Variables & Design Tokens (Light / Dark Mode)
   ========================================================================== */
:root {
  /* Colors */
  --bg-primary: #f8fafc;
  --bg-surface: #ffffff;
  --bg-card: #ffffff;
  --bg-subtle: #f1f5f9;
  
  --text-main: #0f172a;
  --text-muted: #64748b;
  --text-light: #94a3b8;

  --primary: #3b82f6;
  --primary-hover: #2563eb;
  --primary-soft: #eff6ff;

  --accent-exam: #ef4444;
  --accent-exam-bg: #fee2e2;
  --accent-exam-text: #b91c1c;

  --border-color: #e2e8f0;
  --card-shadow: 0 4px 6px -1px rgba(0, 0, 0, 0.05), 0 2px 4px -2px rgba(0, 0, 0, 0.05);
  --card-hover-shadow: 0 10px 15px -3px rgba(0, 0, 0, 0.08), 0 4px 6px -4px rgba(0, 0, 0, 0.04);
  
  --radius-sm: 6px;
  --radius-md: 10px;
  --radius-lg: 16px;
  --radius-full: 9999px;

  --transition-fast: 0.2s ease;
  --transition-normal: 0.3s ease;
}

/* Dark Theme overrides */
body.dark-mode {
  --bg-primary: #0b0f19;
  --bg-surface: #111827;
  --bg-card: #1f2937;
  --bg-subtle: #1e293b;

  --text-main: #f3f4f6;
  --text-muted: #9ca3af;
  --text-light: #6b7280;

  --primary: #60a5fa;
  --primary-hover: #3b82f6;
  --primary-soft: #1e293b;

  --accent-exam: #f87171;
  --accent-exam-bg: #450a0a;
  --accent-exam-text: #fca5a5;

  --border-color: #374151;
  --card-shadow: 0 4px 6px -1px rgba(0, 0, 0, 0.3);
  --card-hover-shadow: 0 10px 20px -3px rgba(0, 0, 0, 0.5);
}

/* ==========================================================================
   Base & Reset
   ========================================================================== */
* {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}

html {
  scroll-behavior: smooth;
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Oxygen, Ubuntu, Cantarell, "Helvetica Neue", sans-serif;
  color: var(--text-main);
  background-color: var(--bg-primary);
  transition: background-color var(--transition-normal), color var(--transition-normal);
}

body {
  line-height: 1.6;
  min-height: 100vh;
  display: flex;
  flex-direction: column;
}

main {
  flex: 1;
}

a {
  text-decoration: none;
  color: inherit;
}

ul {
  list-style: none;
}

.container {
  max-width: 1200px;
  margin: 0 auto;
  padding: 0 1.5rem;
}

.section {
  padding: 4.5rem 0;
}

/* ==========================================================================
   Typography & Shared Components
   ========================================================================== */
.section-header {
  text-align: center;
  max-width: 650px;
  margin: 0 auto 3rem auto;
}

.section-title {
  font-size: 2.1rem;
  font-weight: 700;
  margin-bottom: 0.5rem;
  letter-spacing: -0.02em;
}

.section-subtitle {
  color: var(--text-muted);
  font-size: 1.05rem;
}

.btn {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  font-size: 0.95rem;
  font-weight: 600;
  padding: 0.65rem 1.4rem;
  border-radius: var(--radius-md);
  cursor: pointer;
  border: 1px solid transparent;
  transition: all var(--transition-fast);
}

.btn-primary {
  background-color: var(--primary);
  color: #ffffff;
}

.btn-primary:hover {
  background-color: var(--primary-hover);
  transform: translateY(-1px);
}

.btn-outline {
  border-color: var(--border-color);
  background: var(--bg-surface);
  color: var(--text-main);
}

.btn-outline:hover {
  background: var(--bg-subtle);
  border-color: var(--text-muted);
}

/* ==========================================================================
   Header & Navigation
   ========================================================================== */
.header {
  position: sticky;
  top: 0;
  background-color: var(--bg-surface);
  border-bottom: 1px solid var(--border-color);
  z-index: 1000;
  transition: background-color var(--transition-normal), border-color var(--transition-normal);
}

.nav-container {
  max-width: 1200px;
  margin: 0 auto;
  padding: 1rem 1.5rem;
  display: flex;
  align-items: center;
  justify-content: space-between;
}

.logo {
  display: flex;
  align-items: center;
  gap: 0.5rem;
  font-size: 1.35rem;
  font-weight: 700;
}

.accent-text {
  color: var(--primary);
}

.nav-menu {
  display: flex;
  align-items: center;
  gap: 2rem;
}

.nav-link {
  font-size: 0.95rem;
  font-weight: 500;
  color: var(--text-muted);
  transition: color var(--transition-fast);
}

.nav-link:hover {
  color: var(--primary);
}

.nav-controls {
  display: flex;
  align-items: center;
  gap: 0.75rem;
}

.btn-theme {
  background: var(--bg-subtle);
  border: 1px solid var(--border-color);
  padding: 0.5rem;
  border-radius: var(--radius-md);
  font-size: 1.1rem;
  cursor: pointer;
  line-height: 1;
  transition: background var(--transition-fast);
}

.btn-theme:hover {
  background: var(--border-color);
}

.mobile-toggle {
  display: none;
  flex-direction: column;
  gap: 4px;
  background: transparent;
  border: none;
  cursor: pointer;
  padding: 0.25rem;
}

.mobile-toggle span {
  width: 22px;
  height: 2px;
  background-color: var(--text-main);
  transition: all var(--transition-fast);
}

/* ==========================================================================
   Hero Section
   ========================================================================== */
.hero-section {
  padding: 6rem 0 4.5rem 0;
  background: linear-gradient(180deg, var(--bg-surface) 0%, var(--bg-primary) 100%);
  text-align: center;
}

.hero-badge {
  display: inline-block;
  padding: 0.35rem 0.9rem;
  border-radius: var(--radius-full);
  background: var(--primary-soft);
  color: var(--primary);
  font-size: 0.85rem;
  font-weight: 600;
  margin-bottom: 1.25rem;
}

.hero-title {
  font-size: 3.25rem;
  font-weight: 800;
  line-height: 1.15;
  letter-spacing: -0.03em;
  margin-bottom: 1.2rem;
}

.gradient-text {
  color: var(--primary);
}

.hero-subtitle {
  max-width: 580px;
  margin: 0 auto 2.25rem auto;
  font-size: 1.15rem;
  color: var(--text-muted);
}

.hero-actions {
  display: flex;
  justify-content: center;
  gap: 1rem;
  flex-wrap: wrap;
}

/* ==========================================================================
   Subject Cards
   ========================================================================== */
.subject-grid {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(320px, 1fr));
  gap: 1.5rem;
}

.subject-card {
  background: var(--bg-card);
  border: 1px solid var(--border-color);
  border-radius: var(--radius-lg);
  padding: 1.75rem;
  box-shadow: var(--card-shadow);
  display: flex;
  flex-direction: column;
  justify-content: space-between;
  transition: transform var(--transition-fast), box-shadow var(--transition-fast);
}

.subject-card:hover {
  transform: translateY(-4px);
  box-shadow: var(--card-hover-shadow);
}

.subject-card-icon {
  font-size: 2.2rem;
  margin-bottom: 1rem;
}

.subject-card-title {
  font-size: 1.25rem;
  font-weight: 700;
  margin-bottom: 0.5rem;
}

.subject-card-desc {
  font-size: 0.92rem;
  color: var(--text-muted);
  margin-bottom: 1.5rem;
  flex-grow: 1;
}

.subject-card-footer {
  display: flex;
  align-items: center;
  justify-content: space-between;
  border-top: 1px solid var(--border-color);
  padding-top: 1rem;
}

.note-count-badge {
  font-size: 0.8rem;
  font-weight: 600;
  color: var(--text-muted);
}

/* ==========================================================================
   Search & Filter Section
   ========================================================================== */
.notes-section {
  background-color: var(--bg-surface);
}

.search-filter-wrapper {
  max-width: 800px;
  margin: 0 auto 1.5rem auto;
  display: flex;
  flex-direction: column;
  gap: 1rem;
}

.search-box {
  position: relative;
  display: flex;
  align-items: center;
}

.search-icon {
  position: absolute;
  left: 1rem;
  font-size: 1rem;
  pointer-events: none;
  opacity: 0.6;
}

.search-box input {
  width: 100%;
  padding: 0.85rem 2.8rem 0.85rem 2.8rem;
  border-radius: var(--radius-md);
  border: 1px solid var(--border-color);
  background: var(--bg-primary);
  color: var(--text-main);
  font-size: 0.95rem;
  outline: none;
  transition: border-color var(--transition-fast), box-shadow var(--transition-fast);
}

.search-box input:focus {
  border-color: var(--primary);
  box-shadow: 0 0 0 3px var(--primary-soft);
}

.btn-clear {
  position: absolute;
  right: 0.75rem;
  background: none;
  border: none;
  font-size: 1.25rem;
  color: var(--text-muted);
  cursor: pointer;
  display: none;
}

.btn-clear.show {
  display: block;
}

.filter-pills {
  display: flex;
  flex-wrap: wrap;
  gap: 0.5rem;
  justify-content: center;
}

.filter-pill {
  border: 1px solid var(--border-color);
  background: var(--bg-primary);
  color: var(--text-muted);
  font-size: 0.85rem;
  font-weight: 500;
  padding: 0.35rem 0.9rem;
  border-radius: var(--radius-full);
  cursor: pointer;
  transition: all var(--transition-fast);
}

.filter-pill:hover {
  background: var(--bg-subtle);
  color: var(--text-main);
}

.filter-pill.active {
  background: var(--primary);
  border-color: var(--primary);
  color: #ffffff;
}

.results-meta {
  max-width: 800px;
  margin: 0 auto 2rem auto;
  font-size: 0.88rem;
  color: var(--text-muted);
  text-align: right;
}

/* ==========================================================================
   Notes Cards Grid
   ========================================================================== */
.notes-grid {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(340px, 1fr));
  gap: 1.75rem;
}

.note-card {
  background: var(--bg-card);
  border: 1px solid var(--border-color);
  border-radius: var(--radius-lg);
  padding: 1.75rem;
  box-shadow: var(--card-shadow);
  display: flex;
  flex-direction: column;
  transition: transform var(--transition-fast), box-shadow var(--transition-fast);
}

.note-card:hover {
  box-shadow: var(--card-hover-shadow);
}

.note-card-header {
  display: flex;
  justify-content: space-between;
  align-items: flex-start;
  gap: 0.5rem;
  margin-bottom: 0.75rem;
}

.note-subject-tag {
  font-size: 0.75rem;
  font-weight: 600;
  text-transform: uppercase;
  letter-spacing: 0.05em;
  color: var(--primary);
  background: var(--primary-soft);
  padding: 0.2rem 0.6rem;
  border-radius: var(--radius-sm);
}

.exam-badge {
  font-size: 0.75rem;
  font-weight: 700;
  background-color: var(--accent-exam-bg);
  color: var(--accent-exam-text);
  border: 1px solid var(--accent-exam);
  padding: 0.2rem 0.55rem;
  border-radius: var(--radius-sm);
  display: inline-flex;
  align-items: center;
  gap: 0.25rem;
}

.note-topic-title {
  font-size: 1.25rem;
  font-weight: 700;
  line-height: 1.3;
  margin-bottom: 0.75rem;
}

.note-explanation {
  font-size: 0.93rem;
  color: var(--text-muted);
  margin-bottom: 1.25rem;
}

.note-section-title {
  font-size: 0.85rem;
  font-weight: 700;
  text-transform: uppercase;
  letter-spacing: 0.04em;
  margin-bottom: 0.5rem;
  color: var(--text-main);
}

.note-points-list {
  list-style: disc;
  padding-left: 1.25rem;
  margin-bottom: 1.25rem;
  font-size: 0.9rem;
  color: var(--text-muted);
}

.note-points-list li {
  margin-bottom: 0.4rem;
}

.note-definition-box {
  background: var(--bg-subtle);
  border-left: 3px solid var(--primary);
  border-radius: 0 var(--radius-sm) var(--radius-sm) 0;
  padding: 0.75rem 1rem;
  margin-top: auto;
}

.note-definition-label {
  font-size: 0.75rem;
  font-weight: 700;
  color: var(--primary);
  text-transform: uppercase;
  margin-bottom: 0.2rem;
}

.note-definition-text {
  font-size: 0.88rem;
  font-style: italic;
  color: var(--text-main);
}

/* Empty / Fallback State */
.no-results {
  text-align: center;
  padding: 4rem 1rem;
}

.no-results-icon {
  font-size: 3rem;
  display: block;
  margin-bottom: 1rem;
}

.no-results h3 {
  font-size: 1.4rem;
  margin-bottom: 0.5rem;
}

.no-results p {
  color: var(--text-muted);
  margin-bottom: 1.5rem;
}

.hidden {
  display: none !important;
}

/* ==========================================================================
   About Section
   ========================================================================== */
.about-card {
  background: var(--bg-card);
  border: 1px solid var(--border-color);
  border-radius: var(--radius-lg);
  padding: 3rem;
  display: grid;
  grid-template-columns: 1.5fr 1fr;
  gap: 3rem;
  align-items: center;
  box-shadow: var(--card-shadow);
}

.about-content h3 {
  font-size: 1.75rem;
  margin-bottom: 1rem;
}

.about-content p {
  color: var(--text-muted);
  font-size: 1rem;
}

.about-stats {
  display: grid;
  grid-template-columns: repeat(3, 1fr);
  gap: 1.5rem;
  text-align: center;
}

.stat-item {
  display: flex;
  flex-direction: column;
}

.stat-number {
  font-size: 2rem;
  font-weight: 800;
  color: var(--primary);
}

.stat-label {
  font-size: 0.8rem;
  color: var(--text-muted);
  font-weight: 600;
  text-transform: uppercase;
}

/* ==========================================================================
   Footer
   ========================================================================== */
.footer {
  background-color: var(--bg-surface);
  border-top: 1px solid var(--border-color);
  padding: 4rem 0 2rem 0;
  transition: background-color var(--transition-normal), border-color var(--transition-normal);
}

.footer-grid {
  display: grid;
  grid-template-columns: 2fr 1fr 1fr;
  gap: 3rem;
  margin-bottom: 3rem;
}

.footer-desc {
  color: var(--text-muted);
  font-size: 0.92rem;
  margin-top: 0.75rem;
  max-width: 320px;
}

.footer-links h4 {
  font-size: 0.95rem;
  font-weight: 700;
  margin-bottom: 1rem;
  color: var(--text-main);
}

.footer-links ul {
  display: flex;
  flex-direction: column;
  gap: 0.5rem;
}

.footer-links a {
  color: var(--text-muted);
  font-size: 0.9rem;
  transition: color var(--transition-fast);
}

.footer-links a:hover {
  color: var(--primary);
}

.footer-bottom {
  text-align: center;
  border-top: 1px solid var(--border-color);
  padding-top: 2rem;
  font-size: 0.85rem;
  color: var(--text-muted);
}

/* ==========================================================================
   Responsive Media Queries
   ========================================================================== */
@media (max-width: 868px) {
  .hero-title {
    font-size: 2.5rem;
  }

  .about-card {
    grid-template-columns: 1fr;
    gap: 2rem;
    padding: 2rem;
  }

  .footer-grid {
    grid-template-columns: 1fr;
    gap: 2rem;
  }
}

@media (max-width: 768px) {
  .mobile-toggle {
    display: flex;
  }

  .nav-menu {
    position: absolute;
    top: 100%;
    left: 0;
    width: 100%;
    background-color: var(--bg-surface);
    border-bottom: 1px solid var(--border-color);
    flex-direction: column;
    padding: 1.5rem;
    gap: 1.25rem;
    display: none;
    box-shadow: 0 10px 15px -3px rgba(0, 0, 0, 0.1);
  }

  .nav-menu.open {
    display: flex;
  }
}

@media (max-width: 480px) {
  .hero-title {
    font-size: 2rem;
  }

  .notes-grid,
  .subject-grid {
    grid-template-columns: 1fr;
  }

  .about-stats {
    grid-template-columns: 1fr;
    gap: 1rem;
  }
}