Enhance Flask Sudoku Game UI and Functionality

[AnushaGBhat]

Summary

Enhanced the Flask Sudoku game with the remaining required features and UI improvements.

Changes made

• Added a Hint button with locked/read-only hinted cells and hint tracking
• Added a Check button to highlight incorrect entries without clearing them
• Added a visible countdown timer that resets for each new game or difficulty change
• Added a Top 10 leaderboard using localStorage
• Added dark mode and light mode support
• Added alternating 3×3 region shading
• Improved responsive layout for desktop and mobile devices

Validation

• Ran the complete test suite
• All 16 tests passed successfully

Test Result

16 passed