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/components/EventList.vue

@@ -65,6 +65,11 @@ const deleteDialogMessage = computed(() => {
 
 function requestDelete(eventToken: string) {
   deleteError.value = ''
+  const role = getRole(getStoredEvents().find((e) => e.eventToken === eventToken)!)
+  if (role === 'watcher') {
+    removeEvent(eventToken)
+    return
+  }
   pendingDeleteToken.value = eventToken
 }
 
@@ -105,10 +110,10 @@ function cancelDelete() {
   pendingDeleteToken.value = null
 }
 
-function getRole(event: StoredEvent): 'organizer' | 'attendee' | undefined {
+function getRole(event: StoredEvent): 'organizer' | 'attendee' | 'watcher' {
   if (event.organizerToken) return 'organizer'
   if (event.rsvpToken) return 'attendee'
-  return undefined
+  return 'watcher'
 }
 
 const groupedSections = computed(() => {