feat: improve guestbook UX and add POV indicator
- add drop shadow to guestbook avatar button
- show authenticated user's avatar in sign button (not page owner's)
- add confirmation modal for unauthenticated users: "are you @handle?"
- implement global guestbook signatures cache (all users, not per-DID)
- invalidate cache on sign/delete operations
- return handle and avatar in /api/auth/status
- add "point of view: @handle" indicator on left side with neon styling
🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude <noreply@anthropic.com>