Add client-side watch/bookmark functionality: users can save events to
localStorage without RSVPing via a bookmark button next to the "I'm attending"
CTA. Watched events appear in the event list with a "Watching" label.
Bookmark is only visible for visitors (not attendees or organizers).
Includes spec, plan, research, tasks, unit tests, and E2E tests.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
