On game start and restart the camera
kept the previous run's sky colour, so a fresh desert run after reaching
snow/space showed the wrong sky. Additionally, FIT-scale letterbox bands
showed a fixed white container background, making non-desert biomes appear
to only cover part of the screen with visible top/bottom bands.
- Extract shared LEVEL_BG_COLORS map and reset camera background + level to
desert in startGame() and restartGame(), emitting LEVEL_CHANGED.
- Sync the #game-container DOM background to the active biome colour in
phaser-game.tsx so letterbox bands blend with the sky.