:root {
  --primary: #6366f1;
  --primary-light: #818cf8;
  --primary-dark: #4338ca;
  --secondary: #f59e0b;
  --success: #22c55e;
  --success-bg: #dcfce7;
  --error: #ef4444;
  --error-bg: #fee2e2;
  --bg: #f1f5f9;
  --card: #ffffff;
  --text: #1e293b;
  --text-light: #64748b;
  --border: #e2e8f0;
  --math-color: #6366f1;
  --radius: 12px;
  --shadow: 0 2px 8px rgba(0,0,0,0.08);
  --shadow-lg: 0 8px 24px rgba(0,0,0,0.12);
}

* { box-sizing: border-box; margin: 0; padding: 0; }

body {
  font-family: 'Segoe UI', system-ui, -apple-system, sans-serif;
  background: var(--bg);
  color: var(--text);
  min-height: 100vh;
}

/* ── NAV ───────────────────────────────────── */
nav {
  background: var(--primary-dark);
  padding: 12px 24px;
  display: flex;
  align-items: center;
  gap: 12px;
  flex-wrap: wrap;
}
nav a {
  color: rgba(255,255,255,0.8);
  text-decoration: none;
  font-size: 0.9rem;
  transition: color .2s;
}
nav a:hover { color: #fff; }
nav .sep { color: rgba(255,255,255,0.4); }
nav .current { color: #fff; font-weight: 600; }

/* ── HERO HEADER ───────────────────────────── */
.hero {
  background: linear-gradient(135deg, var(--primary-dark) 0%, var(--primary) 60%, var(--primary-light) 100%);
  color: white;
  padding: 48px 24px 40px;
  text-align: center;
}
.hero h1 { font-size: clamp(1.6rem, 4vw, 2.4rem); font-weight: 800; margin-bottom: 8px; }
.hero p  { font-size: 1.05rem; opacity: .85; max-width: 560px; margin: 0 auto; }

/* ── MAIN CONTAINER ────────────────────────── */
.container { max-width: 960px; margin: 0 auto; padding: 32px 16px; }

/* ── CARDS ─────────────────────────────────── */
.card {
  background: var(--card);
  border-radius: var(--radius);
  box-shadow: var(--shadow);
  padding: 28px;
  margin-bottom: 24px;
}

/* ── LEVEL CARDS (home page) ───────────────── */
.level-grid {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(260px, 1fr));
  gap: 24px;
  margin-top: 40px;
}
.level-card {
  background: var(--card);
  border-radius: 16px;
  box-shadow: var(--shadow-lg);
  padding: 40px 32px;
  text-align: center;
  text-decoration: none;
  color: var(--text);
  transition: transform .2s, box-shadow .2s;
  border: 2px solid transparent;
}
.level-card:hover {
  transform: translateY(-4px);
  box-shadow: 0 16px 40px rgba(0,0,0,0.16);
  border-color: var(--primary);
}
.level-card .emoji { font-size: 3.5rem; margin-bottom: 16px; }
.level-card h2 { font-size: 1.6rem; font-weight: 800; margin-bottom: 8px; }
.level-card p  { color: var(--text-light); font-size: 0.95rem; }

/* ── TOPIC GRID ────────────────────────────── */
.topic-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(220px, 1fr));
  gap: 16px;
}
.topic-card {
  background: var(--card);
  border-radius: var(--radius);
  box-shadow: var(--shadow);
  padding: 20px;
  text-decoration: none;
  color: var(--text);
  transition: transform .15s, box-shadow .15s;
  border: 2px solid transparent;
  position: relative;
}
.topic-card:hover { transform: translateY(-2px); box-shadow: var(--shadow-lg); border-color: var(--primary); }
.topic-card .topic-icon { font-size: 1.8rem; margin-bottom: 10px; }
.topic-card h3 { font-size: 1rem; font-weight: 700; margin-bottom: 6px; }
.topic-card .topic-sub { font-size: 0.82rem; color: var(--text-light); }
.topic-card .done-badge {
  position: absolute; top: 12px; right: 12px;
  background: var(--success); color: #fff;
  border-radius: 50%; width: 24px; height: 24px;
  display: flex; align-items: center; justify-content: center;
  font-size: 0.8rem; font-weight: 700;
}

/* ── PROGRESS BAR ──────────────────────────── */
.progress-section { margin-bottom: 32px; }
.progress-label { display: flex; justify-content: space-between; margin-bottom: 8px; font-weight: 600; }
.progress-bar-outer { background: var(--border); border-radius: 99px; height: 14px; overflow: hidden; }
.progress-bar-inner { height: 100%; background: linear-gradient(90deg, var(--primary), var(--primary-light)); border-radius: 99px; transition: width .4s ease; }

/* ── EXERCISE SECTIONS ─────────────────────── */
.exercise-section {
  background: var(--card);
  border-radius: var(--radius);
  box-shadow: var(--shadow);
  margin-bottom: 28px;
  overflow: hidden;
}
.exercise-header {
  background: linear-gradient(90deg, var(--primary-dark), var(--primary));
  color: white;
  padding: 14px 24px;
  font-size: 1rem;
  font-weight: 700;
}
.exercise-body { padding: 24px; }

.question { margin-bottom: 18px; line-height: 1.6; }
.question p { margin-bottom: 8px; }
.question strong { color: var(--primary-dark); }

/* ── INPUTS ────────────────────────────────── */
.answer-line { display: flex; align-items: center; gap: 8px; margin: 8px 0; flex-wrap: wrap; }
.answer-input {
  border: 2px solid var(--border);
  border-radius: 8px;
  padding: 6px 12px;
  font-size: 1rem;
  font-family: inherit;
  min-width: 80px;
  width: auto;
  transition: border-color .2s;
}
.answer-input:focus { outline: none; border-color: var(--primary); }
.answer-input.correct { border-color: var(--success); background: var(--success-bg); }
.answer-input.incorrect { border-color: var(--error); background: var(--error-bg); }
.feedback-icon { font-size: 1.1rem; min-width: 20px; }

textarea.answer-input {
  width: 100%; min-height: 80px; resize: vertical;
  font-size: 0.95rem; line-height: 1.5;
}

/* ── BUTTONS ───────────────────────────────── */
.btn {
  display: inline-flex; align-items: center; gap: 6px;
  padding: 10px 20px; border-radius: 8px; border: none;
  font-size: 0.95rem; font-weight: 600; cursor: pointer;
  transition: transform .15s, box-shadow .15s, background .15s;
  text-decoration: none;
}
.btn:hover { transform: translateY(-1px); box-shadow: 0 4px 12px rgba(0,0,0,0.15); }
.btn-primary { background: var(--primary); color: white; }
.btn-primary:hover { background: var(--primary-dark); }
.btn-success { background: var(--success); color: white; }
.btn-outline { background: white; color: var(--primary); border: 2px solid var(--primary); }
.btn-outline:hover { background: var(--primary); color: white; }
.btn-secondary { background: var(--secondary); color: white; }
.btn-secondary:hover { background: #d97706; }

/* ── CORRECTION BOX ────────────────────────── */
.correction-box {
  background: #fefce8;
  border: 2px solid #fbbf24;
  border-radius: var(--radius);
  padding: 20px;
  margin-top: 16px;
  display: none;
}
.correction-box.visible { display: block; }
.correction-box h4 { color: #92400e; margin-bottom: 12px; font-size: 1rem; }
.correction-box p, .correction-box li { color: #451a03; line-height: 1.7; }
.correction-box ul { padding-left: 20px; }

/* ── PAGE ACTIONS ──────────────────────────── */
.page-actions {
  display: flex; align-items: center; gap: 12px; flex-wrap: wrap;
  margin-top: 32px; padding-top: 24px;
  border-top: 2px solid var(--border);
}
.done-toggle { display: flex; align-items: center; gap: 8px; cursor: pointer; font-weight: 600; }
.done-toggle input[type=checkbox] { width: 20px; height: 20px; accent-color: var(--success); cursor: pointer; }
.done-toggle.checked { color: var(--success); }

/* ── TABLE ─────────────────────────────────── */
table { border-collapse: collapse; width: 100%; margin: 12px 0; }
th, td { border: 1px solid var(--border); padding: 8px 12px; text-align: center; font-size: 0.9rem; }
th { background: #f8fafc; font-weight: 700; }

/* ── LINK ──────────────────────────────────── */
.resource-link {
  display: inline-flex; align-items: center; gap: 8px;
  background: linear-gradient(90deg, #1e40af, var(--primary));
  color: white; padding: 12px 20px; border-radius: 10px;
  text-decoration: none; font-weight: 600;
  transition: opacity .2s;
}
.resource-link:hover { opacity: .88; }

/* ── PLACEHOLDER ───────────────────────────── */
.placeholder {
  text-align: center; padding: 60px 20px;
  color: var(--text-light);
}
.placeholder .emoji { font-size: 4rem; margin-bottom: 16px; }

/* ── VERSION BADGE ─────────────────────────── */
body::after {
  content: "v1.2";
  position: fixed;
  bottom: 10px;
  right: 12px;
  font-size: .72rem;
  font-weight: 600;
  color: #94a3b8;
  background: rgba(255,255,255,.85);
  border: 1px solid #e2e8f0;
  border-radius: 20px;
  padding: 2px 8px;
  pointer-events: none;
  z-index: 9000;
  letter-spacing: .04em;
}

/* ── RESPONSIVE ────────────────────────────── */
@media (max-width: 600px) {
  .hero { padding: 32px 16px; }
  .card { padding: 20px; }
  .exercise-body { padding: 16px; }
  .topic-grid { grid-template-columns: 1fr 1fr; }
}