Add RSVP frontend: bottom sheet form, RsvpBar, and localStorage persistence

Introduces BottomSheet and RsvpBar components, integrates the RSVP
submission flow into EventDetailView, extends useEventStorage with
saveRsvp/getRsvp, and adds unit tests plus an E2E spec for the RSVP
workflow.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/composables/__tests__/useEventStorage.spec.ts

@@ -116,4 +116,52 @@ describe('useEventStorage', () => {
     expect(events).toHaveLength(1)
     expect(events[0]!.title).toBe('New Title')
   })
+
+  it('saves and retrieves RSVP for an existing event', () => {
+    const { saveCreatedEvent, saveRsvp, getRsvp } = useEventStorage()
+
+    saveCreatedEvent({
+      eventToken: 'abc-123',
+      title: 'Birthday',
+      dateTime: '2026-06-15T20:00:00+02:00',
+      expiryDate: '2026-07-15',
+    })
+
+    saveRsvp('abc-123', 'rsvp-token-1', 'Max', 'Birthday', '2026-06-15T20:00:00+02:00')
+
+    const rsvp = getRsvp('abc-123')
+    expect(rsvp).toEqual({ rsvpToken: 'rsvp-token-1', rsvpName: 'Max' })
+  })
+
+  it('saves RSVP for a new event (not previously stored)', () => {
+    const { saveRsvp, getRsvp, getStoredEvents } = useEventStorage()
+
+    saveRsvp('new-event', 'rsvp-token-2', 'Anna', 'Party', '2026-08-01T18:00:00+02:00')
+
+    const rsvp = getRsvp('new-event')
+    expect(rsvp).toEqual({ rsvpToken: 'rsvp-token-2', rsvpName: 'Anna' })
+
+    const events = getStoredEvents()
+    expect(events).toHaveLength(1)
+    expect(events[0]!.eventToken).toBe('new-event')
+    expect(events[0]!.title).toBe('Party')
+  })
+
+  it('returns undefined RSVP for event without RSVP', () => {
+    const { saveCreatedEvent, getRsvp } = useEventStorage()
+
+    saveCreatedEvent({
+      eventToken: 'abc-123',
+      title: 'Test',
+      dateTime: '2026-06-15T20:00:00+02:00',
+      expiryDate: '2026-07-15',
+    })
+
+    expect(getRsvp('abc-123')).toBeUndefined()
+  })
+
+  it('returns undefined RSVP for unknown event', () => {
+    const { getRsvp } = useEventStorage()
+    expect(getRsvp('unknown')).toBeUndefined()
+  })
 })