Fix Game Over screen does not appear when no moves are left in 2048 game

[yuvraj-k-singh]

📝 Description

The 2048 game fails to display the "Game Over" message when the board is completely filled and no valid moves are left.

Currently, the check_game_over() function is only called after a successful move. If the player presses an arrow key and no tiles move, the game-over condition is never checked, causing the game to remain stuck without ending properly.

Additionally, multiple "GAME OVER" labels can be created if the function is triggered repeatedly.

🔗 Linked Issue

Closes #628

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📋 Contribution Checklist

• My code strictly adheres to the project guidelines (Standard library only, procedural code, clean spacing).
• I have verified that my files are placed in the correct directory.
• I have tested my changes thoroughly on my local machine.
• I have included interactive emojis and clean console/UI outputs.
• GSSoC 2026: I have been formally assigned to this issue and noted it above.

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[github-actions]

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