The board was invisible because:
- board.js sets `this.el.className = 'board'` (8x8 grid)
- chessboard.css .board uses `width:100%; height:100%` — needs a sized parent
- The CSS expects a `.board-wrapper` parent with padding-top:100% trick
- I was using a custom `.board-container` with aspect-ratio which doesn't work

Fixed: all game screens now use `.board-wrapper > #board` structure
which chessboard.css already handles (position:relative + padding-top:100%
for the wrapper, position:absolute + inset:0 for the board grid).
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>