Add cancel RSVP feature (backend DELETE endpoint + frontend UI)

Allows guests to cancel their RSVP via a DELETE endpoint using their
guestToken. Frontend shows cancel button in RsvpBar and clears local
storage on success. Includes unit tests, integration tests, and E2E spec.

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

frontend/src/composables/useEventStorage.ts

@@ -78,10 +78,20 @@ export function useEventStorage() {
     return undefined
   }
 
+  function removeRsvp(eventToken: string): void {
+    const events = readEvents()
+    const event = events.find((e) => e.eventToken === eventToken)
+    if (event) {
+      delete event.rsvpToken
+      delete event.rsvpName
+      writeEvents(events)
+    }
+  }
+
   function removeEvent(eventToken: string): void {
     const events = readEvents().filter((e) => e.eventToken !== eventToken)
     writeEvents(events)
   }
 
-  return { saveCreatedEvent, getStoredEvents, getOrganizerToken, saveRsvp, getRsvp, removeEvent }
+  return { saveCreatedEvent, getStoredEvents, getOrganizerToken, saveRsvp, getRsvp, removeRsvp, removeEvent }
 }