/* styles.css */
:root {
    --primary-color: #3b82f6;
    --secondary-color: #6b7280;
    --success-color: #10b981;
    --warning-color: #f59e0b;
    --error-color: #ef4444;
    --background: #f8fafc;
    --surface: #ffffff;
    --text-primary: #1f2937;
    --text-secondary: #6b7280;
    --border: #e5e7eb;
    --shadow: 0 4px 6px -1px rgba(0, 0, 0, 0.1);
  }
  
  @media (prefers-color-scheme: dark) {
    :root {
      --background: #0f172a;
      --surface: #1e293b;
      --text-primary: #f1f5f9;
      --text-secondary: #94a3b8;
      --border: #334155;
    }
  }
  
  * {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
  }
  
  body {
    font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, sans-serif;
    background-color: var(--background);
    color: var(--text-primary);
    line-height: 1.6;
    min-height: 100vh;
  }
  
  .container {
    max-width: 800px;
    margin: 0 auto;
    padding: 1rem;
    min-height: 100vh;
    display: flex;
    flex-direction: column;
  }
  
  .header {
    text-align: center;
    margin-bottom: 2rem;
    padding: 1rem 0;
    border-bottom: 1px solid var(--border);
  }
  
  .header h1 {
    font-size: 2rem;
    font-weight: 700;
    margin-bottom: 1rem;
    color: var(--primary-color);
  }
  
  .stats {
    display: flex;
    justify-content: center;
    gap: 2rem;
    flex-wrap: wrap;
  }
  
  .stats span {
    font-weight: 600;
    color: var(--text-secondary);
  }
  
  .loading {
    display: flex;
    flex-direction: column;
    align-items: center;
    justify-content: center;
    flex: 1;
    gap: 1rem;
  }
  
  .spinner {
    width: 40px;
    height: 40px;
    border: 4px solid var(--border);
    border-top: 4px solid var(--primary-color);
    border-radius: 50%;
    animation: spin 1s linear infinite;
  }
  
  @keyframes spin {
    0% { transform: rotate(0deg); }
    100% { transform: rotate(360deg); }
  }
  
  .hidden {
    display: none !important;
  }
  
  .app {
    flex: 1;
    display: flex;
    flex-direction: column;
    gap: 2rem;
  }
  
  .controls {
    display: flex;
    justify-content: center;
    gap: 1rem;
    flex-wrap: wrap;
  }
  
  .level-select {
    padding: 0.5rem 1rem;
    border: 1px solid var(--border);
    border-radius: 0.5rem;
    background: var(--surface);
    color: var(--text-primary);
    font-size: 1rem;
  }
  
  .btn {
    padding: 0.75rem 1.5rem;
    border: none;
    border-radius: 0.5rem;
    font-size: 1rem;
    font-weight: 600;
    cursor: pointer;
    transition: all 0.2s;
    text-align: center;
    min-width: 80px;
  }
  
  .btn:hover {
    transform: translateY(-1px);
    box-shadow: var(--shadow);
  }
  
  .btn:active {
    transform: translateY(0);
  }
  
  .btn-primary {
    background: var(--primary-color);
    color: white;
  }
  
  .btn-secondary {
    background: var(--secondary-color);
    color: white;
  }
  
  .btn-unknown {
    background: var(--error-color);
    color: white;
    flex: 1;
  }
  
  .btn-known {
    background: var(--success-color);
    color: white;
    flex: 1;
  }
  
  .card-container {
    display: flex;
    justify-content: center;
    flex: 1;
    align-items: center;
  }
  
  .flashcard {
    background: var(--surface);
    border-radius: 1rem;
    box-shadow: var(--shadow);
    width: 100%;
    max-width: 500px;
    min-height: 400px;
    padding: 2rem;
    display: flex;
    flex-direction: column;
    justify-content: center;
    text-align: center;
    position: relative;
  }
  
  .card-front,
  .card-back {
    display: flex;
    flex-direction: column;
    justify-content: center;
    align-items: center;
    gap: 2rem;
    min-height: 300px;
  }
  
  .chinese {
    font-size: 4rem;
    font-weight: 700;
    color: var(--text-primary);
    margin-bottom: 1rem;
  }
  
  .pinyin {
    font-size: 1.5rem;
    color: var(--text-secondary);
    margin-bottom: 2rem;
  }
  
  .meaning {
    font-size: 2rem;
    font-weight: 600;
    color: var(--text-primary);
    margin-bottom: 1rem;
    line-height: 1.4;
  }
  
  .additional-info {
    display: flex;
    flex-direction: column;
    gap: 0.5rem;
    margin-bottom: 2rem;
  }
  
  .pos,
  .frequency {
    font-size: 0.9rem;
    color: var(--text-secondary);
  }
  
  .answer-buttons {
    display: flex;
    gap: 1rem;
    width: 100%;
    max-width: 300px;
    justify-content: center;
  }
  
  .session-stats {
    display: flex;
    justify-content: center;
    gap: 2rem;
    padding: 1rem;
    background: var(--surface);
    border-radius: 0.5rem;
    flex-wrap: wrap;
  }
  
  .stat {
    display: flex;
    flex-direction: column;
    align-items: center;
    gap: 0.25rem;
  }
  
  .stat-label {
    font-size: 0.875rem;
    color: var(--text-secondary);
  }
  
  .stat span:last-child {
    font-size: 1.25rem;
    font-weight: 700;
    color: var(--primary-color);
  }
  
  .keyboard-hints {
    text-align: center;
    margin-top: 1rem;
    padding: 1rem;
    background: var(--surface);
    border-radius: 0.5rem;
    font-size: 0.875rem;
    color: var(--text-secondary);
    line-height: 1.6;
  }
  
  .keyboard-hints p {
    margin-bottom: 0.5rem;
  }
  
  .keyboard-hints p:last-child {
    margin-bottom: 0;
  }
  
  .keyboard-hints kbd {
    background: var(--border);
    color: var(--text-primary);
    padding: 0.25rem 0.5rem;
    border-radius: 0.25rem;
    font-family: monospace;
    font-size: 0.75rem;
    margin: 0 0.125rem;
    border: 1px solid var(--text-secondary);
  }
  
  /* Mobile optimizations */
  @media (max-width: 640px) {
    .container {
      padding: 0.5rem;
    }
    
    .header h1 {
      font-size: 1.5rem;
    }
    
    .stats {
      gap: 1rem;
    }
    
    .chinese {
      font-size: 3rem;
    }
    
    .meaning {
      font-size: 1.5rem;
    }
    
    .flashcard {
      padding: 1.5rem;
      min-height: 350px;
    }
    
    .answer-buttons {
      flex-direction: column;
      gap: 0.75rem;
    }
    
    .session-stats {
      gap: 1rem;
    }
    
    .keyboard-hints {
      display: none;
    }
  }
  
  /* Keyboard navigation */
  .btn:focus,
  .level-select:focus {
    outline: 2px solid var(--primary-color);
    outline-offset: 2px;
  }
  
  /* Animations */
  .flashcard {
    transition: transform 0.3s ease;
  }
  
  .flashcard:hover {
    transform: translateY(-2px);
  }
  
  /* Accessibility */
  @media (prefers-reduced-motion: reduce) {
    .spinner {
      animation: none;
    }
    
    .btn:hover {
      transform: none;
    }
    
    .flashcard:hover {
      transform: none;
    }
  }
  
  /* Error state */
  .error {
    display: flex;
    flex-direction: column;
    align-items: center;
    gap: 1rem;
    color: var(--error-color);
  }
  
  .error p {
    font-size: 1.125rem;
    font-weight: 600;
  }
  
  /* Dark mode specific adjustments */
  @media (prefers-color-scheme: dark) {
    .keyboard-hints kbd {
      background: var(--text-secondary);
      color: var(--background);
      border-color: var(--text-primary);
    }
  }