feat(client): build GameBoard.tscn with full node hierarchy (spec B1.2)
- BoardCamera, BoardTileMap, GhostTileMap, HighlightTileMap
- UILayer (CanvasLayer layer=1): HUD, TileHand, MeeplePicker
- FXLayer as Node2D world space so FX follow camera zoom/pan
- Scripts attached, signals connected, initial visibility set