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/components/__tests__/EventCard.spec.ts

@@ -63,6 +63,12 @@ describe('EventCard', () => {
     expect(wrapper.text()).toContain('Attendee')
   })
 
+  it('renders watcher badge when eventRole is watcher', () => {
+    const wrapper = mountCard({ eventRole: 'watcher' })
+    expect(wrapper.find('.event-card__badge--watcher').exists()).toBe(true)
+    expect(wrapper.text()).toContain('Watching')
+  })
+
   it('renders no badge when eventRole is undefined', () => {
     const wrapper = mountCard({ eventRole: undefined })
     expect(wrapper.find('.event-card__badge').exists()).toBe(false)