.fifteen-board {
  display: grid;
  grid-template-columns: repeat(4, minmax(0, 1fr));
  gap: 8px;
  width: min(380px, 100%);
  border: 1px solid var(--hairline);
  border-radius: var(--radius-lg);
  padding: 10px;
  background: var(--canvas);
  box-shadow: var(--shadow-sm);
}

.fifteen-tile {
  aspect-ratio: 1;
  border: 1px solid var(--hairline-strong);
  border-radius: var(--radius-sm);
  background: linear-gradient(180deg, #ffffff, #f1f5f9);
  box-shadow: var(--shadow-sm);
  color: var(--ink);
  font-size: clamp(20px, 5vw, 28px);
  font-weight: 600;
  line-height: 1;
  cursor: pointer;
  transition:
    transform 120ms ease,
    border-color 120ms ease,
    box-shadow 120ms ease;
}

.fifteen-tile:hover,
.fifteen-tile:focus-visible {
  transform: translateY(-1px);
  border-color: var(--ink);
  box-shadow: var(--shadow-md);
}

.fifteen-tile.is-blank {
  border: 1px dashed var(--hairline-cool);
  background: transparent;
  box-shadow: none;
  cursor: default;
}

.fifteen-tile.is-blank:hover {
  transform: none;
  border-color: var(--hairline-cool);
  box-shadow: none;
}

.fifteen-tile.is-correct {
  border-color: rgba(62, 207, 142, 0.6);
  background: linear-gradient(180deg, rgba(62, 207, 142, 0.16), rgba(62, 207, 142, 0.08));
  color: var(--primary-deep);
}