Remove unused landing page CSS

The about page was removed in commit deca919
but its CSS styles remained. This removes the now-unused landing page,
feature card, stats grid, and other styles that were only used by the
about page.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <[email protected]>

Author: jonathanhefner