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/views/__tests__/EventCreateView.spec.ts

@@ -14,6 +14,8 @@ vi.mock('@/composables/useEventStorage', () => ({
     saveCreatedEvent: vi.fn(),
     getStoredEvents: vi.fn(() => []),
     getOrganizerToken: vi.fn(),
+    saveRsvp: vi.fn(),
+    getRsvp: vi.fn(),
   })),
 }))
 
@@ -165,6 +167,8 @@ describe('EventCreateView', () => {
       saveCreatedEvent: mockSave,
       getStoredEvents: vi.fn(() => []),
       getOrganizerToken: vi.fn(),
+      saveRsvp: vi.fn(),
+      getRsvp: vi.fn(),
     })
 
     vi.mocked(api.POST).mockResolvedValueOnce({