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()
+  })
 })

frontend/src/composables/useEventStorage.ts

@@ -4,6 +4,8 @@ export interface StoredEvent {
   title: string
   dateTime: string
   expiryDate: string
+  rsvpToken?: string
+  rsvpName?: string
 }
 
 const STORAGE_KEY = 'fete:events'
@@ -37,5 +39,25 @@ export function useEventStorage() {
     return event?.organizerToken
   }
 
-  return { saveCreatedEvent, getStoredEvents, getOrganizerToken }
+  function saveRsvp(eventToken: string, rsvpToken: string, rsvpName: string, title: string, dateTime: string): void {
+    const events = readEvents()
+    const existing = events.find((e) => e.eventToken === eventToken)
+    if (existing) {
+      existing.rsvpToken = rsvpToken
+      existing.rsvpName = rsvpName
+    } else {
+      events.push({ eventToken, title, dateTime, expiryDate: '', rsvpToken, rsvpName })
+    }
+    writeEvents(events)
+  }
+
+  function getRsvp(eventToken: string): { rsvpToken: string; rsvpName: string } | undefined {
+    const event = readEvents().find((e) => e.eventToken === eventToken)
+    if (event?.rsvpToken && event?.rsvpName) {
+      return { rsvpToken: event.rsvpToken, rsvpName: event.rsvpName }
+    }
+    return undefined
+  }
+
+  return { saveCreatedEvent, getStoredEvents, getOrganizerToken, saveRsvp, getRsvp }
 }