feat(client): build all three scenes from scratch (spec B1.2/B1.4/B1.6)
- 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