Implement Game Over Detection

[bhumishah2411]

Detect when no valid moves remain and show “Game Over” message or modal.

Acceptance Criteria:

• Check no empty cells
• Check no possible merges
• Show “Game Over” message/modal
• Provide "New Game" button

[harshit-sharma2005]

HI @bhumishah2411 I would love to contribute to the issue. Kindly assign it to me.

[vaswzerdali]

@bhumishah2411 hello! I would really much appreciate if you assign me to contribute to this issue!