Fix projects page CSS glitches and layout issues

- Updated HTML classes from introduction-expertise-* to github-* for consistency
- Fixed CSS class mismatches between HTML and stylesheets
- Added topic class to span elements in repo-topics
- Changed button classes from neo-matrix-btn to repo-link
- Updated JavaScript selectors to match new class names
- Added Matrix theme background color directly to body
- Added DOMContentLoaded handler to ensure proper page loading
- Fixed tab filtering and overall page layout

The projects page now displays correctly with the Matrix theme and all interactive elements work as expected.

Author: ThomasJButler