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/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 }
 }