:root {
  --primary: #4F46E5;
  --primary-light: #818CF8;
  --primary-dark: #3730A3;
  --success: #10B981;
  --danger: #EF4444;
  --warning: #F59E0B;
  --bg: #F3F4F6;
  --card: #FFFFFF;
  --text: #1F2937;
  --text-secondary: #6B7280;
  --border: #E5E7EB;
  --radius: 12px;
}

* { margin: 0; padding: 0; box-sizing: border-box; }
body { font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', 'PingFang SC', 'Microsoft YaHei', sans-serif; background: var(--bg); color: var(--text); line-height: 1.6; min-height: 100vh; }
a { color: var(--primary); text-decoration: none; }

/* Layout */
.app-container { max-width: 960px; margin: 0 auto; padding: 20px; }
.header { background: white; border-bottom: 1px solid var(--border); padding: 12px 24px; display: flex; align-items: center; justify-content: space-between; position: sticky; top: 0; z-index: 100; }
.header h1 { font-size: 18px; color: var(--primary); font-weight: 700; }
.header .user-info { display: flex; align-items: center; gap: 12px; font-size: 14px; color: var(--text-secondary); }

/* Cards */
.card { background: var(--card); border-radius: var(--radius); box-shadow: 0 1px 3px rgba(0,0,0,0.08); padding: 24px; margin-bottom: 20px; }
.card-title { font-size: 16px; font-weight: 600; margin-bottom: 16px; color: var(--text); }

/* Buttons */
.btn { display: inline-flex; align-items: center; justify-content: center; gap: 6px; padding: 10px 20px; border: none; border-radius: 8px; font-size: 14px; font-weight: 500; cursor: pointer; transition: all 0.2s; }
.btn-primary { background: var(--primary); color: white; }
.btn-primary:hover { background: var(--primary-dark); }
.btn-success { background: var(--success); color: white; }
.btn-danger { background: var(--danger); color: white; }
.btn-outline { background: transparent; border: 1px solid var(--border); color: var(--text); }
.btn-outline:hover { background: var(--bg); }
.btn-sm { padding: 6px 14px; font-size: 13px; }
.btn:disabled { opacity: 0.5; cursor: not-allowed; }
.btn-block { width: 100%; }

/* Forms */
.form-group { margin-bottom: 16px; }
.form-group label { display: block; font-size: 14px; font-weight: 500; margin-bottom: 6px; color: var(--text); }
.form-input { width: 100%; padding: 10px 14px; border: 1px solid var(--border); border-radius: 8px; font-size: 14px; transition: border-color 0.2s; outline: none; font-family: inherit; }
.form-input:focus { border-color: var(--primary); box-shadow: 0 0 0 3px rgba(79,70,229,0.1); }
textarea.form-input { resize: vertical; min-height: 120px; }

/* Login */
.login-container { display: flex; align-items: center; justify-content: center; min-height: 100vh; padding: 20px; }
.login-box { width: 100%; max-width: 400px; }
.login-box .logo { text-align: center; margin-bottom: 32px; }
.login-box .logo h2 { font-size: 24px; color: var(--primary); margin-top: 12px; }
.login-box .logo p { color: var(--text-secondary); font-size: 14px; margin-top: 4px; }

/* Upload Area */
.upload-area { border: 2px dashed var(--border); border-radius: var(--radius); padding: 40px 20px; text-align: center; cursor: pointer; transition: all 0.2s; }
.upload-area:hover, .upload-area.dragover { border-color: var(--primary); background: rgba(79,70,229,0.03); }
.upload-area .icon { font-size: 40px; margin-bottom: 12px; }
.upload-area p { color: var(--text-secondary); font-size: 14px; }
.upload-area .formats { font-size: 12px; color: var(--text-secondary); margin-top: 8px; }

/* Tabs */
.tabs { display: flex; gap: 0; border-bottom: 2px solid var(--border); margin-bottom: 20px; }
.tab { padding: 10px 20px; cursor: pointer; font-size: 14px; font-weight: 500; color: var(--text-secondary); border-bottom: 2px solid transparent; margin-bottom: -2px; transition: all 0.2s; }
.tab.active { color: var(--primary); border-bottom-color: var(--primary); }
.tab:hover { color: var(--text); }

/* Steps */
.step-card { border: 1px solid var(--border); border-radius: var(--radius); padding: 20px; margin-bottom: 16px; position: relative; }
.step-header { display: flex; align-items: center; gap: 10px; margin-bottom: 12px; }
.step-number { width: 28px; height: 28px; border-radius: 50%; background: var(--primary); color: white; display: flex; align-items: center; justify-content: center; font-size: 13px; font-weight: 600; flex-shrink: 0; }
.step-title { font-weight: 600; font-size: 15px; }
.step-content { font-size: 14px; line-height: 1.8; white-space: pre-wrap; }

/* Blanks */
.blank-input { display: inline-block; min-width: 80px; max-width: 200px; padding: 4px 10px; border: 2px solid var(--primary-light); border-radius: 6px; font-size: 14px; text-align: center; background: rgba(79,70,229,0.05); outline: none; font-family: inherit; vertical-align: middle; }
.blank-input:focus { border-color: var(--primary); background: white; box-shadow: 0 0 0 3px rgba(79,70,229,0.1); }
.blank-input.correct { border-color: var(--success); background: rgba(16,185,129,0.08); color: var(--success); }
.blank-input.incorrect { border-color: var(--danger); background: rgba(239,68,68,0.08); color: var(--danger); }

/* Final answer */
.final-answer-section { border: 2px solid var(--primary); border-radius: var(--radius); padding: 20px; margin-top: 20px; background: rgba(79,70,229,0.02); }
.final-answer-section .label { font-weight: 600; color: var(--primary); margin-bottom: 8px; font-size: 15px; }

/* Results */
.result-banner { padding: 20px; border-radius: var(--radius); text-align: center; margin-bottom: 20px; }
.result-banner.good { background: rgba(16,185,129,0.1); border: 1px solid var(--success); }
.result-banner.bad { background: rgba(239,68,68,0.1); border: 1px solid var(--danger); }
.result-banner .score { font-size: 36px; font-weight: 700; }
.result-banner .detail { font-size: 14px; color: var(--text-secondary); margin-top: 4px; }
.correction { font-size: 13px; color: var(--danger); margin-top: 4px; }
.correction .correct-ans { color: var(--success); font-weight: 600; }

/* History */
.history-item { display: flex; justify-content: space-between; align-items: center; padding: 12px 0; border-bottom: 1px solid var(--border); }
.history-item:last-child { border-bottom: none; }
.history-meta { font-size: 13px; color: var(--text-secondary); }
.badge { display: inline-block; padding: 2px 10px; border-radius: 12px; font-size: 12px; font-weight: 500; }
.badge-success { background: rgba(16,185,129,0.1); color: var(--success); }
.badge-warning { background: rgba(245,158,11,0.1); color: var(--warning); }
.badge-info { background: rgba(79,70,229,0.1); color: var(--primary); }

/* Loading */
.loading { display: flex; flex-direction: column; align-items: center; justify-content: center; padding: 60px 20px; }
.spinner { width: 40px; height: 40px; border: 3px solid var(--border); border-top-color: var(--primary); border-radius: 50%; animation: spin 0.8s linear infinite; }
@keyframes spin { to { transform: rotate(360deg); } }
.loading p { margin-top: 16px; color: var(--text-secondary); font-size: 14px; }

/* Toast */
.toast { position: fixed; top: 20px; right: 20px; padding: 12px 20px; border-radius: 8px; color: white; font-size: 14px; z-index: 9999; animation: slideIn 0.3s; }
.toast-success { background: var(--success); }
.toast-error { background: var(--danger); }
@keyframes slideIn { from { transform: translateX(100%); opacity: 0; } to { transform: translateX(0); opacity: 1; } }

/* Responsive */
@media (max-width: 640px) {
  .app-container { padding: 12px; }
  .card { padding: 16px; }
  .header { padding: 10px 16px; }
  .header h1 { font-size: 16px; }
  .upload-area { padding: 24px 16px; }
}

/* Utility */
.hidden { display: none !important; }
.text-center { text-align: center; }
.mt-2 { margin-top: 8px; }
.mt-4 { margin-top: 16px; }
.mb-4 { margin-bottom: 16px; }
.flex { display: flex; }
.gap-2 { gap: 8px; }
.gap-4 { gap: 16px; }
.items-center { align-items: center; }
.justify-between { justify-content: space-between; }