Implement watch-event feature (017) with bookmark in RsvpBar

Add client-side watch/bookmark functionality: users can save events to
localStorage without RSVPing via a bookmark button next to the "I'm attending"
CTA. Watched events appear in the event list with a "Watching" label.
Bookmark is only visible for visitors (not attendees or organizers).

Includes spec, plan, research, tasks, unit tests, and E2E tests.

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

frontend/src/composables/useEventStorage.ts

@@ -88,10 +88,24 @@ export function useEventStorage() {
     }
   }
 
+  function saveWatch(eventToken: string, title: string, dateTime: string): void {
+    const events = readEvents()
+    const existing = events.find((e) => e.eventToken === eventToken)
+    if (!existing) {
+      events.push({ eventToken, title, dateTime })
+      writeEvents(events)
+    }
+  }
+
+  function isStored(eventToken: string): boolean {
+    void version.value
+    return readEvents().some((e) => e.eventToken === eventToken)
+  }
+
   function removeEvent(eventToken: string): void {
     const events = readEvents().filter((e) => e.eventToken !== eventToken)
     writeEvents(events)
   }
 
-  return { saveCreatedEvent, getStoredEvents, getOrganizerToken, saveRsvp, getRsvp, removeRsvp, removeEvent }
+  return { saveCreatedEvent, getStoredEvents, getOrganizerToken, saveRsvp, getRsvp, removeRsvp, saveWatch, isStored, removeEvent }
 }