Align styling with overlake.bio reference site

- Update overlake-styles.css and particles-config.js to the refined
  reference versions (design tokens, mellower easing, scroll-reveal,
  callout/skip-link, print styles)
- Vendor particles.min.js locally and add enhancements.js (scroll-reveal);
  drop the CDN particles.js dependency
- Add WebP background and og-image.png social card
- Rewrite index.html head with full meta/OG tags scoped to ai.overlake.bio,
  add skip-link, switch to self-hosted scripts

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: raimbetov