Keep background in sync with active level biome

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.