Add UI support for Restart and Quit functionality

The current pong game doesn't include any on-screen options to restart or quit. Users have to manually refresh the browser or close the tab to restart the game, which isn't ideal.

Proposed Enhancements:
Restart Button

Add a button labeled Restart that resets the game state (score, ball position, paddle positions) and starts a new game without reloading the page.

Quit Button

Add a Quit or Exit button that stops the game loop and optionally shows a “Game Over” or “Thanks for playing!” screen.

Implementation Ideas:
Use HTML elements styled with CSS, or draw custom buttons directly on the canvas.

Attach onclick event listeners to trigger a game reset or stop the animation loop.

For “Quit”, consider pausing the game and showing a message overlay instead of just closing the window (since browsers don't allow scripts to close tabs easily).