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/composables/__tests__/useEventStorage.spec.ts

@@ -194,6 +194,71 @@ describe('useEventStorage', () => {
   })
 })
 
+describe('useEventStorage – saveWatch / isStored', () => {
+  beforeEach(() => {
+    clearStorage()
+  })
+
+  it('saves a watch-only event (no rsvpToken, no organizerToken)', () => {
+    const { saveWatch, getStoredEvents } = useEventStorage()
+
+    saveWatch('watch-1', 'Concert', '2026-07-01T20:00:00+02:00')
+
+    const events = getStoredEvents()
+    expect(events).toHaveLength(1)
+    expect(events[0]!.eventToken).toBe('watch-1')
+    expect(events[0]!.title).toBe('Concert')
+    expect(events[0]!.dateTime).toBe('2026-07-01T20:00:00+02:00')
+    expect(events[0]!.rsvpToken).toBeUndefined()
+    expect(events[0]!.organizerToken).toBeUndefined()
+  })
+
+  it('does not duplicate if event already stored', () => {
+    const { saveWatch, saveRsvp, getStoredEvents } = useEventStorage()
+
+    saveRsvp('evt-1', 'rsvp-1', 'Max', 'Party', '2026-07-01T20:00:00+02:00')
+    saveWatch('evt-1', 'Party', '2026-07-01T20:00:00+02:00')
+
+    expect(getStoredEvents()).toHaveLength(1)
+    expect(getStoredEvents()[0]!.rsvpToken).toBe('rsvp-1')
+  })
+
+  it('isStored returns true for watched events', () => {
+    const { saveWatch, isStored } = useEventStorage()
+
+    saveWatch('watch-1', 'Concert', '2026-07-01T20:00:00+02:00')
+
+    expect(isStored('watch-1')).toBe(true)
+  })
+
+  it('isStored returns true for attended events', () => {
+    const { saveRsvp, isStored } = useEventStorage()
+
+    saveRsvp('evt-1', 'rsvp-1', 'Max', 'Party', '2026-07-01T20:00:00+02:00')
+
+    expect(isStored('evt-1')).toBe(true)
+  })
+
+  it('isStored returns true for organized events', () => {
+    const { saveCreatedEvent, isStored } = useEventStorage()
+
+    saveCreatedEvent({
+      eventToken: 'evt-1',
+      organizerToken: 'org-1',
+      title: 'My Event',
+      dateTime: '2026-07-01T20:00:00+02:00',
+    })
+
+    expect(isStored('evt-1')).toBe(true)
+  })
+
+  it('isStored returns false for unknown tokens', () => {
+    const { isStored } = useEventStorage()
+
+    expect(isStored('unknown')).toBe(false)
+  })
+})
+
 describe('isValidStoredEvent', () => {
   // Import directly since it's an exported function
   let isValidStoredEvent: (e: unknown) => boolean