[Feature] Add menu music to StartScreen and LevelSelect

[alimaandev]

Problem\nThe StartScreen and LevelSelect are silent. Music only plays during 3D story levels.\n\nSolution\nWire startMusic() into Game.tsx so ambient/drone music plays on:\n- StartScreen (screen === 'start')\n- LevelSelect (screen === 'levelselect')\n- GameOverScreen (screen === 'gameover') — low intensity ambient\n\nCall stopMusic() when transitioning to playing mode to avoid overlap.\n\nFile: src/components/Game.tsx, src/game/audio.ts