/* CSS Variables */
:root {
  --primary: #4f46e5;
  --primary-dark: #4338ca;
  --primary-light: #818cf8;
  --secondary: #10b981;
  --secondary-dark: #059669;
  --warning: #f59e0b;
  --danger: #ef4444;
  --success: #22c55e;

  --bg-primary: #ffffff;
  --bg-secondary: #f9fafb;
  --bg-tertiary: #f3f4f6;

  --text-primary: #111827;
  --text-secondary: #6b7280;
  --text-muted: #9ca3af;

  --border-color: #e5e7eb;
  --shadow-sm: 0 1px 2px rgba(0, 0, 0, 0.05);
  --shadow-md: 0 4px 6px -1px rgba(0, 0, 0, 0.1);
  --shadow-lg: 0 10px 15px -3px rgba(0, 0, 0, 0.1);

  --radius-sm: 0.375rem;
  --radius-md: 0.5rem;
  --radius-lg: 0.75rem;
  --radius-xl: 1rem;

  --transition: all 0.2s ease;
}

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

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

body {
  font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, 'Helvetica Neue', Arial, sans-serif;
  background: var(--bg-secondary);
  color: var(--text-primary);
  line-height: 1.6;
  min-height: 100vh;
}

/* Typography */
h1, h2, h3, h4, h5, h6 {
  font-weight: 600;
  line-height: 1.3;
}

h1 { font-size: 2rem; }
h2 { font-size: 1.5rem; }
h3 { font-size: 1.25rem; }
h4 { font-size: 1.125rem; }

a {
  color: var(--primary);
  text-decoration: none;
}

a:hover {
  color: var(--primary-dark);
}

/* Layout */
.container {
  max-width: 1200px;
  margin: 0 auto;
  padding: 0 1rem;
}

.page-wrapper {
  display: flex;
  min-height: 100vh;
}

.sidebar {
  width: 250px;
  background: var(--bg-primary);
  border-right: 1px solid var(--border-color);
  padding: 1.5rem;
  position: fixed;
  height: 100vh;
  overflow-y: auto;
}

.main-content {
  flex: 1;
  margin-left: 250px;
  padding: 2rem;
}

/* Navigation */
.logo {
  display: flex;
  align-items: center;
  gap: 0.75rem;
  font-size: 1.5rem;
  font-weight: 700;
  color: var(--primary);
  margin-bottom: 2rem;
}

.logo-icon {
  font-size: 2rem;
}

.nav-section {
  margin-bottom: 1.5rem;
}

.nav-section-title {
  font-size: 0.75rem;
  font-weight: 600;
  text-transform: uppercase;
  letter-spacing: 0.05em;
  color: var(--text-muted);
  margin-bottom: 0.75rem;
}

.nav-link {
  display: flex;
  align-items: center;
  gap: 0.75rem;
  padding: 0.75rem 1rem;
  border-radius: var(--radius-md);
  color: var(--text-secondary);
  transition: var(--transition);
  margin-bottom: 0.25rem;
}

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

.nav-link.active {
  background: var(--primary);
  color: white;
}

.nav-icon {
  font-size: 1.25rem;
}

/* User Info */
.user-info {
  padding: 1rem;
  background: var(--bg-tertiary);
  border-radius: var(--radius-md);
  margin-top: auto;
}

.user-name {
  font-weight: 600;
  margin-bottom: 0.25rem;
}

.user-stats {
  display: flex;
  gap: 1rem;
  font-size: 0.875rem;
  color: var(--text-secondary);
}

/* Cards */
.card {
  background: var(--bg-primary);
  border-radius: var(--radius-lg);
  box-shadow: var(--shadow-sm);
  border: 1px solid var(--border-color);
  padding: 1.5rem;
  margin-bottom: 1.5rem;
}

.card-header {
  display: flex;
  justify-content: space-between;
  align-items: center;
  margin-bottom: 1rem;
}

.card-title {
  font-size: 1.125rem;
  font-weight: 600;
}

/* Buttons */
.btn {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  gap: 0.5rem;
  padding: 0.625rem 1.25rem;
  font-size: 0.875rem;
  font-weight: 500;
  border-radius: var(--radius-md);
  border: none;
  cursor: pointer;
  transition: var(--transition);
}

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

.btn-primary:hover {
  background: var(--primary-dark);
}

.btn-secondary {
  background: var(--bg-tertiary);
  color: var(--text-primary);
}

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

.btn-success {
  background: var(--success);
  color: white;
}

.btn-warning {
  background: var(--warning);
  color: white;
}

.btn-danger {
  background: var(--danger);
  color: white;
}

.btn-outline {
  background: transparent;
  border: 1px solid var(--border-color);
  color: var(--text-primary);
}

.btn-outline:hover {
  background: var(--bg-tertiary);
}

.btn-sm {
  padding: 0.375rem 0.75rem;
  font-size: 0.75rem;
}

.btn-lg {
  padding: 0.875rem 1.75rem;
  font-size: 1rem;
}

.btn-block {
  width: 100%;
}

/* Forms */
.form-group {
  margin-bottom: 1.25rem;
}

.form-label {
  display: block;
  font-size: 0.875rem;
  font-weight: 500;
  margin-bottom: 0.5rem;
  color: var(--text-primary);
}

.form-input {
  width: 100%;
  padding: 0.75rem 1rem;
  font-size: 1rem;
  border: 1px solid var(--border-color);
  border-radius: var(--radius-md);
  background: var(--bg-primary);
  transition: var(--transition);
}

.form-input:focus {
  outline: none;
  border-color: var(--primary);
  box-shadow: 0 0 0 3px rgba(79, 70, 229, 0.1);
}

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

textarea.form-input {
  min-height: 100px;
  resize: vertical;
}

.form-hint {
  font-size: 0.75rem;
  color: var(--text-muted);
  margin-top: 0.25rem;
}

/* Search */
.search-box {
  position: relative;
}

.search-box .form-input {
  padding-left: 2.5rem;
}

.search-icon {
  position: absolute;
  left: 0.875rem;
  top: 50%;
  transform: translateY(-50%);
  color: var(--text-muted);
}

/* Badges/Tags */
.badge {
  display: inline-flex;
  align-items: center;
  padding: 0.25rem 0.625rem;
  font-size: 0.75rem;
  font-weight: 500;
  border-radius: 9999px;
  background: var(--bg-tertiary);
  color: var(--text-secondary);
}

.badge-primary {
  background: rgba(79, 70, 229, 0.1);
  color: var(--primary);
}

.badge-success {
  background: rgba(34, 197, 94, 0.1);
  color: var(--success);
}

.badge-warning {
  background: rgba(245, 158, 11, 0.1);
  color: var(--warning);
}

/* Stats Grid */
.stats-grid {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(200px, 1fr));
  gap: 1rem;
}

.stat-card {
  background: var(--bg-primary);
  border-radius: var(--radius-lg);
  border: 1px solid var(--border-color);
  padding: 1.25rem;
  text-align: center;
}

.stat-value {
  font-size: 2rem;
  font-weight: 700;
  color: var(--primary);
}

.stat-label {
  font-size: 0.875rem;
  color: var(--text-secondary);
  margin-top: 0.25rem;
}

/* Progress Bar */
.progress-bar {
  width: 100%;
  height: 0.5rem;
  background: var(--bg-tertiary);
  border-radius: 9999px;
  overflow: hidden;
}

.progress-fill {
  height: 100%;
  background: var(--primary);
  border-radius: 9999px;
  transition: width 0.3s ease;
}

.progress-fill.success {
  background: var(--success);
}

/* Lists */
.list {
  list-style: none;
}

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

.list-item:last-child {
  border-bottom: none;
}

.list-item:hover {
  background: var(--bg-secondary);
}

/* Word Card */
.word-card {
  background: var(--bg-primary);
  border-radius: var(--radius-lg);
  border: 1px solid var(--border-color);
  padding: 1.25rem;
  margin-bottom: 1rem;
  transition: var(--transition);
}

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

.word-title {
  font-size: 1.25rem;
  font-weight: 600;
  margin-bottom: 0.5rem;
}

.word-translation {
  color: var(--primary);
  font-weight: 500;
}

.word-definition {
  color: var(--text-secondary);
  margin-top: 0.5rem;
}

.word-tags {
  display: flex;
  flex-wrap: wrap;
  gap: 0.5rem;
  margin-top: 0.75rem;
}

.word-actions {
  display: flex;
  gap: 0.5rem;
  margin-top: 1rem;
}

/* Flashcard */
.flashcard {
  background: var(--bg-primary);
  border-radius: var(--radius-xl);
  box-shadow: var(--shadow-lg);
  width: 100%;
  max-width: 500px;
  min-height: 300px;
  margin: 2rem auto;
  perspective: 1000px;
  cursor: pointer;
}

.flashcard-inner {
  position: relative;
  width: 100%;
  height: 100%;
  min-height: 300px;
  transition: transform 0.6s;
  transform-style: preserve-3d;
}

.flashcard.flipped .flashcard-inner {
  transform: rotateY(180deg);
}

.flashcard-front, .flashcard-back {
  position: absolute;
  width: 100%;
  height: 100%;
  min-height: 300px;
  backface-visibility: hidden;
  display: flex;
  align-items: center;
  justify-content: center;
  padding: 2rem;
  font-size: 1.5rem;
  text-align: center;
  border-radius: var(--radius-xl);
}

.flashcard-front {
  background: var(--bg-primary);
}

.flashcard-back {
  background: var(--primary);
  color: white;
  transform: rotateY(180deg);
}

/* Review Buttons */
.review-buttons {
  display: flex;
  justify-content: center;
  gap: 0.5rem;
  flex-wrap: wrap;
  margin-top: 1.5rem;
}

.review-btn {
  padding: 0.75rem 1.5rem;
  font-size: 0.875rem;
  font-weight: 600;
  border-radius: var(--radius-md);
  border: none;
  cursor: pointer;
  transition: var(--transition);
}

.review-btn.again {
  background: var(--danger);
  color: white;
}

.review-btn.hard {
  background: var(--warning);
  color: white;
}

.review-btn.good {
  background: var(--success);
  color: white;
}

.review-btn.easy {
  background: var(--primary);
  color: white;
}

.review-btn:hover {
  transform: translateY(-2px);
  box-shadow: var(--shadow-md);
}

/* Achievement */
.achievement {
  display: flex;
  align-items: center;
  gap: 1rem;
  padding: 1rem;
  background: var(--bg-primary);
  border-radius: var(--radius-lg);
  border: 1px solid var(--border-color);
  margin-bottom: 0.75rem;
}

.achievement.earned {
  border-color: var(--success);
  background: rgba(34, 197, 94, 0.05);
}

.achievement.locked {
  opacity: 0.5;
}

.achievement-icon {
  font-size: 2.5rem;
}

.achievement-info {
  flex: 1;
}

.achievement-name {
  font-weight: 600;
}

.achievement-desc {
  font-size: 0.875rem;
  color: var(--text-secondary);
}

/* Streak Display */
.streak-display {
  display: flex;
  align-items: center;
  gap: 0.5rem;
  font-size: 1.25rem;
  font-weight: 600;
  color: var(--warning);
}

/* Level Display */
.level-display {
  display: flex;
  align-items: center;
  gap: 1rem;
  margin-bottom: 1rem;
}

.level-badge {
  width: 60px;
  height: 60px;
  background: var(--primary);
  border-radius: 50%;
  display: flex;
  align-items: center;
  justify-content: center;
  font-size: 1.5rem;
  font-weight: 700;
  color: white;
}

.level-info {
  flex: 1;
}

.level-title {
  font-weight: 600;
}

.xp-text {
  font-size: 0.875rem;
  color: var(--text-secondary);
}

/* Grid Layouts */
.grid-2 {
  display: grid;
  grid-template-columns: repeat(2, 1fr);
  gap: 1.5rem;
}

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

.grid-4 {
  display: grid;
  grid-template-columns: repeat(4, 1fr);
  gap: 1rem;
}

/* Modal */
.modal-overlay {
  position: fixed;
  inset: 0;
  background: rgba(0, 0, 0, 0.5);
  display: flex;
  align-items: center;
  justify-content: center;
  z-index: 1000;
  opacity: 0;
  visibility: hidden;
  transition: var(--transition);
}

.modal-overlay.active {
  opacity: 1;
  visibility: visible;
}

.modal {
  background: var(--bg-primary);
  border-radius: var(--radius-xl);
  box-shadow: var(--shadow-lg);
  width: 100%;
  max-width: 500px;
  max-height: 90vh;
  overflow-y: auto;
  transform: scale(0.95);
  transition: var(--transition);
}

.modal-overlay.active .modal {
  transform: scale(1);
}

.modal-header {
  display: flex;
  justify-content: space-between;
  align-items: center;
  padding: 1.25rem 1.5rem;
  border-bottom: 1px solid var(--border-color);
}

.modal-title {
  font-size: 1.125rem;
  font-weight: 600;
}

.modal-close {
  background: none;
  border: none;
  font-size: 1.5rem;
  cursor: pointer;
  color: var(--text-muted);
  padding: 0.25rem;
}

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

.modal-body {
  padding: 1.5rem;
}

.modal-footer {
  display: flex;
  justify-content: flex-end;
  gap: 0.75rem;
  padding: 1rem 1.5rem;
  border-top: 1px solid var(--border-color);
}

/* Tabs */
.tabs {
  display: flex;
  border-bottom: 1px solid var(--border-color);
  margin-bottom: 1.5rem;
}

.tab {
  padding: 0.75rem 1.5rem;
  font-size: 0.875rem;
  font-weight: 500;
  color: var(--text-secondary);
  border-bottom: 2px solid transparent;
  cursor: pointer;
  transition: var(--transition);
}

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

.tab.active {
  color: var(--primary);
  border-bottom-color: var(--primary);
}

/* Activity Heatmap */
.heatmap {
  display: grid;
  grid-template-columns: repeat(53, 12px);
  gap: 3px;
}

.heatmap-day {
  width: 12px;
  height: 12px;
  border-radius: 2px;
  background: var(--bg-tertiary);
}

.heatmap-day.level-1 { background: rgba(79, 70, 229, 0.2); }
.heatmap-day.level-2 { background: rgba(79, 70, 229, 0.4); }
.heatmap-day.level-3 { background: rgba(79, 70, 229, 0.6); }
.heatmap-day.level-4 { background: var(--primary); }

/* Auth Pages */
.auth-page {
  min-height: 100vh;
  display: flex;
  align-items: center;
  justify-content: center;
  background: linear-gradient(135deg, var(--primary) 0%, var(--primary-dark) 100%);
}

.auth-container {
  background: var(--bg-primary);
  border-radius: var(--radius-xl);
  box-shadow: var(--shadow-lg);
  width: 100%;
  max-width: 400px;
  padding: 2.5rem;
}

.auth-logo {
  text-align: center;
  margin-bottom: 2rem;
}

.auth-logo h1 {
  color: var(--primary);
  font-size: 2rem;
}

.auth-title {
  font-size: 1.5rem;
  text-align: center;
  margin-bottom: 1.5rem;
}

.auth-footer {
  text-align: center;
  margin-top: 1.5rem;
  color: var(--text-secondary);
}

/* Toast Notifications */
.toast-container {
  position: fixed;
  bottom: 1.5rem;
  right: 1.5rem;
  z-index: 2000;
}

.toast {
  background: var(--bg-primary);
  border-radius: var(--radius-md);
  box-shadow: var(--shadow-lg);
  padding: 1rem 1.25rem;
  margin-bottom: 0.5rem;
  display: flex;
  align-items: center;
  gap: 0.75rem;
  animation: slideIn 0.3s ease;
}

.toast.success { border-left: 4px solid var(--success); }
.toast.error { border-left: 4px solid var(--danger); }
.toast.warning { border-left: 4px solid var(--warning); }
.toast.info { border-left: 4px solid var(--primary); }

@keyframes slideIn {
  from {
    transform: translateX(100%);
    opacity: 0;
  }
  to {
    transform: translateX(0);
    opacity: 1;
  }
}

/* Empty State */
.empty-state {
  text-align: center;
  padding: 3rem 1rem;
  color: var(--text-muted);
}

.empty-state-icon {
  font-size: 3rem;
  margin-bottom: 1rem;
}

.empty-state-title {
  font-size: 1.25rem;
  font-weight: 600;
  color: var(--text-secondary);
  margin-bottom: 0.5rem;
}

/* Loading Spinner */
.spinner {
  width: 40px;
  height: 40px;
  border: 3px solid var(--border-color);
  border-top-color: var(--primary);
  border-radius: 50%;
  animation: spin 0.8s linear infinite;
}

@keyframes spin {
  to { transform: rotate(360deg); }
}

/* Mind Map Canvas */
.mindmap-container {
  width: 100%;
  height: 600px;
  background: var(--bg-tertiary);
  border-radius: var(--radius-lg);
  position: relative;
  overflow: hidden;
}

.mindmap-canvas {
  width: 100%;
  height: 100%;
}

.mindmap-node {
  position: absolute;
  padding: 0.75rem 1.25rem;
  background: var(--bg-primary);
  border-radius: var(--radius-md);
  box-shadow: var(--shadow-md);
  cursor: move;
  user-select: none;
  min-width: 100px;
  text-align: center;
}

.mindmap-node.root {
  background: var(--primary);
  color: white;
  font-weight: 600;
}

.mindmap-node.selected {
  outline: 2px solid var(--primary);
  outline-offset: 2px;
}

/* Mobile Menu Toggle */
.mobile-menu-btn {
  display: none;
  position: fixed;
  bottom: 1rem;
  right: 1rem;
  width: 56px;
  height: 56px;
  border-radius: 50%;
  background: var(--primary);
  color: white;
  font-size: 1.5rem;
  border: none;
  box-shadow: var(--shadow-lg);
  z-index: 100;
  cursor: pointer;
}

/* Responsive Design */
@media (max-width: 1024px) {
  .grid-3 {
    grid-template-columns: repeat(2, 1fr);
  }

  .grid-4 {
    grid-template-columns: repeat(2, 1fr);
  }
}

@media (max-width: 768px) {
  .sidebar {
    position: fixed;
    left: -250px;
    z-index: 200;
    transition: var(--transition);
  }

  .sidebar.open {
    left: 0;
  }

  .main-content {
    margin-left: 0;
    padding: 1rem;
  }

  .mobile-menu-btn {
    display: flex;
    align-items: center;
    justify-content: center;
  }

  .grid-2, .grid-3, .grid-4 {
    grid-template-columns: 1fr;
  }

  .stats-grid {
    grid-template-columns: repeat(2, 1fr);
  }

  .review-buttons {
    flex-direction: column;
  }

  .review-btn {
    width: 100%;
  }

  .heatmap {
    overflow-x: auto;
  }

  .modal {
    max-width: calc(100% - 2rem);
    margin: 1rem;
  }

  .auth-container {
    margin: 1rem;
  }
}

@media (max-width: 480px) {
  html {
    font-size: 14px;
  }

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

  .flashcard {
    min-height: 250px;
  }

  .flashcard-front, .flashcard-back {
    min-height: 250px;
    font-size: 1.25rem;
  }
}

/* Utility Classes */
.hidden { display: none !important; }
.flex { display: flex; }
.flex-center { display: flex; align-items: center; justify-content: center; }
.flex-between { display: flex; align-items: center; justify-content: space-between; }
.gap-1 { gap: 0.5rem; }
.gap-2 { gap: 1rem; }
.mt-1 { margin-top: 0.5rem; }
.mt-2 { margin-top: 1rem; }
.mb-1 { margin-bottom: 0.5rem; }
.mb-2 { margin-bottom: 1rem; }
.text-center { text-align: center; }
.text-muted { color: var(--text-muted); }
.text-success { color: var(--success); }
.text-danger { color: var(--danger); }
.font-bold { font-weight: 700; }