Implement watch-event feature (017) with bookmark in RsvpBar

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>

frontend/src/views/__tests__/EventCreateView.spec.ts

@@ -164,6 +164,8 @@ describe('EventCreateView', () => {
       saveRsvp: vi.fn(),
       getRsvp: vi.fn(),
       removeRsvp: vi.fn(),
+      saveWatch: vi.fn(),
+      isStored: vi.fn(() => false),
       removeEvent: vi.fn(),
     })