Implement Variable Board Sizes (3x3, 5x5)

[bhumishah2411]

Allow selecting different grid sizes before new game.

Acceptance Criteria:

• UI to choose board size (dropdown or modal)
• Grid renders selected size
• Game logic adapts to any size

[unnati-09m]

Hi @bhumishah2411 I'd like to work on this please assign it to me .