.c4-board {
  display: grid;
  grid-template-columns: repeat(7, 1fr);
  gap: 6px;
  padding: 10px;
  background: #2563eb;
  border-radius: 12px;
  width: min(360px, 92vw);
  margin: 0 auto;
  touch-action: manipulation;
}
.c4-col { display: flex; flex-direction: column; gap: 6px; cursor: pointer; }
.c4-cell {
  width: 100%;
  aspect-ratio: 1 / 1;
  border-radius: 50%;
  background: #eaf1ff;
  box-shadow: inset 0 -3px 0 rgba(0,0,0,.18);
}
.c4-cell.r { background: #ef4444; }
.c4-cell.y { background: #facc15; }
.c4-cell.win { outline: 3px solid #fff; outline-offset: 1px; }