Perf-5: Switch game loop to requestAnimationFrame

[snackman]

Summary

• Replace setInterval with requestAnimationFrame + fixed-timestep accumulator
• Game logic still ticks at 50ms, synced with display refresh
• Clean tab-switch behavior (pause instead of throttled ticks)
• Zero changes to game logic subsystems — only constants.ts and useGameLogic.ts modified

Test plan

• Entity movement speed identical to before
• Tab switching pauses/resumes cleanly
• Pause/resume works
• Oven timers function correctly
• Game over animation works

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