Enhance Game Flow: Game Over Detection, Move Tracking, and Restart Functionality

[BhushanVidhate-27]

This PR improves game logic and UI by introducing:

• Enhanced Game Over Detection:

1. Checks both empty tiles and possible merges.
2. Shows a popup when no moves are possible.

• Move Tracking:

1. All slide functions now return a moved flag.
2. Tiles only move if there’s a valid change, preventing unnecessary new tiles.

• Restart Functionality:

1. Resets the board, score, and messages.
2. Hides the game over popup and creates new initial tiles.

• UI/UX Improvements:

1. Styled game over popup with semi-transparent background.
2. Smooth hover effects on retry button.
3. Proper spacing and alignment consistent with the game's theme.

• Impact:

1. Ensures accurate game state tracking.
2. Adds a more polished game over experience.
3. Prevents invalid moves from affecting the game state.