/* ==========================================================================
   Algebra Balance — styles
   ========================================================================== */

:root {
  --bg: #0f172a;
  --panel: #1e293b;
  --panel-2: #334155;
  --text: #f1f5f9;
  --muted: #94a3b8;
  --x: #60a5fa;
  --num: #34d399;
  --accent: #a78bfa;
  --danger: #f87171;
}

* {
  box-sizing: border-box;
}

body {
  font-family: system-ui, -apple-system, sans-serif;
  background: var(--bg);
  color: var(--text);
  margin: 0;
  padding: 16px;
  min-height: 100vh;
}

.container {
  max-width: 720px;
  margin: 0 auto;
}

/* Header ------------------------------------------------------------------ */

header {
  text-align: center;
  margin-bottom: 14px;
}

header h1 {
  margin: 0 0 4px;
  font-size: 1.6rem;
}

header p {
  margin: 0;
  color: var(--muted);
  font-size: 0.95rem;
}

/* Level selector chips ------------------------------------------------------ */

#levels {
  display: flex;
  flex-wrap: wrap;
  gap: 8px;
  justify-content: center;
  margin: 16px 0;
}

.level-chip {
  border: 1px solid var(--panel-2);
  background: var(--panel);
  color: var(--text);
  border-radius: 999px;
  padding: 8px 14px;
  font-size: 0.9rem;
  font-weight: 600;
  cursor: pointer;
  transition: background 0.2s, transform 0.15s;
}

.level-chip:hover {
  background: var(--panel-2);
}

.level-chip.active {
  background: var(--accent);
  border-color: var(--accent);
}

.level-chip.done {
  border-color: var(--num);
  color: var(--num);
}

.level-chip.done.active {
  color: #fff;
}

.level-chip:active {
  transform: scale(0.96);
}

/* The scale ---------------------------------------------------------------- */

#scale {
  background: var(--panel);
  border: 2px solid var(--panel-2);
  border-radius: 16px;
  padding: 16px 12px;
  display: flex;
  align-items: center;
  justify-content: center;
  gap: 8px;
  min-height: 150px;
}

.side {
  flex: 1 1 0;
  min-width: 0;
  min-height: 110px;
  background: var(--panel-2);
  border-radius: 12px;
  padding: 10px;
  display: flex;
  flex-wrap: wrap;
  gap: 8px;
  align-content: center;
  justify-content: center;
  position: relative;
}

.equals {
  font-size: 2rem;
  font-weight: 800;
  color: var(--muted);
  flex: 0 0 auto;
}

.block {
  padding: 10px 14px;
  border-radius: 8px;
  font-weight: 700;
  font-size: 1.05rem;
  color: #fff;
  display: inline-block;
  position: relative;
  user-select: none;
}

.x-block {
  background: var(--x);
  box-shadow: 0 2px 0 rgba(0, 0, 0, 0.25);
}

.num-block {
  background: var(--num);
  box-shadow: 0 2px 0 rgba(0, 0, 0, 0.25);
}

/* Grouping overlay for ÷ ---------------------------------------------------- */

.group-overlay {
  position: absolute;
  border: 3px dashed #fbbf24;
  border-radius: 12px;
  background: rgba(251, 191, 36, 0.12);
  z-index: 5;
  pointer-events: none;
}

.group-badge {
  position: absolute;
  top: -12px;
  right: -8px;
  background: #f59e0b;
  color: #fff;
  border-radius: 999px;
  padding: 2px 8px;
  font-size: 0.75rem;
  font-weight: 800;
  line-height: 1.4;
  box-shadow: 0 2px 4px rgba(0, 0, 0, 0.3);
}

/* Controls ------------------------------------------------------------------ */

#controls {
  display: flex;
  flex-wrap: wrap;
  gap: 8px;
  justify-content: center;
  margin-top: 16px;
}

button {
  border: none;
  border-radius: 10px;
  padding: 12px 18px;
  font-size: 1rem;
  font-weight: 700;
  color: #fff;
  background: var(--accent);
  cursor: pointer;
  transition: opacity 0.2s, transform 0.15s;
}

button:hover:not(:disabled) {
  filter: brightness(1.1);
}

button:active:not(:disabled) {
  transform: scale(0.97);
}

button:disabled {
  opacity: 0.35;
  cursor: not-allowed;
}

.op-sub {
  background: #ef4444;
}

.op-add {
  background: #10b981;
}

.op-div {
  background: #f59e0b;
}

/* Feedback ------------------------------------------------------------------ */

#feedback {
  min-height: 2rem;
  margin: 14px 0 4px;
  text-align: center;
  font-weight: 700;
  font-size: 1.05rem;
}

#feedback.ok {
  color: var(--num);
}

#feedback.err {
  color: var(--danger);
}

#feedback.info {
  color: var(--accent);
}

.caption {
  text-align: center;
  color: var(--muted);
  font-size: 0.85rem;
  margin-top: 6px;
  min-height: 1.2em;
}

.hint {
  background: var(--panel);
  border-left: 4px solid var(--accent);
  border-radius: 8px;
  padding: 10px 12px;
  margin-top: 14px;
  color: var(--muted);
  font-size: 0.92rem;
}

/* Mobile -------------------------------------------------------------------- */

@media (max-width: 560px) {
  body {
    padding: 10px;
  }

  header h1 {
    font-size: 1.3rem;
  }

  #scale {
    padding: 10px 6px;
    gap: 4px;
  }

  .side {
    min-height: 96px;
    padding: 8px;
    gap: 6px;
  }

  .block {
    padding: 8px 10px;
    font-size: 0.95rem;
  }

  .equals {
    font-size: 1.5rem;
  }

  button {
    padding: 12px 14px;
    font-size: 0.9rem;
    flex: 1 1 40%;
  }

  .level-chip {
    padding: 7px 12px;
    font-size: 0.8rem;
  }
}