Fix 2048 game over logic

[anfernee]

• Added moves_available() to Grid to check if any empty tiles exist or adjacent tiles can be merged.
• Modified Game.prototype.move() to trigger game over when no moves are available.
• Fixed KeyboardManager.remove() to properly delete event listeners from this.events (using delete this.events[event] instead of .delete(), which fails since it's an Object, not a Map).

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PR created automatically by Jules for task 17677514275306089276 started by @anfernee

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