Add animations and visual effects for win/loss states (GSSoC'25)

### Description
Currently, the pong game's UI is very basic with no visual feedback when a player or AI wins. Adding some subtle but engaging visual effects will improve the overall user experience and make the game more lively.

### Proposed Enhancements
- Show a short animation or effect when the user scores a point
- Add visual or audio feedback on game over (win/loss)
- Use CSS transitions or JavaScript-based canvas effects
- Optional: show a scoreboard animation or color change

### Benefits
- More polished game feel
- Encourages replayability and engagement
- Visual feedback helps communicate game events clearly

### Tech Stack
- CSS animations / JavaScript effects
- Audio API (optional)
- Canvas rendering tweaks

I am a GSSoC'25 contributor.

### Request
Please assign this issue to me under GSSoC'25 so i can contribute in it.

Labels Requested- 'gssoc 2025' 'UI' 'good first issue'

No specific files or tests are named. Start by locating the HTML, CSS, and JavaScript entry points and reproducing a scoring and game-over transition; done means win/loss and scoring states have consistent visual feedback without disrupting play, while audio and scoreboard changes remain optional.

索引モデルが issue の本文から書いたものです。