refactor: extract standalone leaderboard CSS and JS into modular assets

refactor: extract standalone leaderboard CSS and JS into modular assets

- move inline CSS from index.html to index/css/style.css
- move inline JavaScript from index.html to index/js/script.js
- add index/images directory for future leaderboard assets
- update index.html to reference external stylesheet and script
- preserve existing UI, responsiveness, animations, and functionality
- improve maintainability and separation of concerns without changing behavior

Closes #32

Author: udaycodespace