.bt-board {
  display: grid;
  grid-template-columns: repeat(8, 44px);
  gap: 0;
  margin: 8px auto;
  width: max-content;
  border: 4px solid #3a4a5e;
  border-radius: 8px;
  overflow: hidden;
}
.bt-cell {
  width: 44px;
  height: 44px;
  position: relative;
  display: flex;
  align-items: center;
  justify-content: center;
  background: #f3f6fb;
}
.bt-cell.odd { background: #e3e9f1; }
.bt-cell.sel { box-shadow: inset 0 0 0 3px #ffd93d; }
.bt-cell.dot::after {
  content: "";
  width: 13px; height: 13px;
  border-radius: 50%;
  background: rgba(40,160,80,.65);
  position: absolute;
}
.bt-piece {
  width: 32px; height: 32px;
  border-radius: 50%;
  box-shadow: 0 2px 3px rgba(0,0,0,.3), inset 0 2px 4px rgba(255,255,255,.35);
}
.bt-piece.r { background: radial-gradient(circle at 35% 30%, #ff7b7b, #d63031); }
.bt-piece.b { background: radial-gradient(circle at 35% 30%, #5a6b7d, #2d3a4a); }