fix link previews, mobile layout, and /@handle routing
- remove static OG tags from app.html so SSR tags take precedence
- add /@handle route that resolves to /profile/{did}
- update internal links to use /@handle format
- mobile-first CSS: header overflow, touch-friendly buttons, responsive sizing
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>