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/EventCard.vue

@@ -12,7 +12,7 @@
       <span class="event-card__time">{{ displayTime }}</span>
     </RouterLink>
     <span v-if="eventRole" class="event-card__badge" :class="`event-card__badge--${eventRole}`">
-      {{ eventRole === 'organizer' ? 'Organizer' : 'Attendee' }}
+      {{ eventRole === 'organizer' ? 'Organizer' : eventRole === 'attendee' ? 'Attendee' : 'Watching' }}
     </span>
     <button
       class="event-card__delete"
@@ -34,7 +34,7 @@ const props = defineProps<{
   title: string
   relativeTime: string
   isPast: boolean
-  eventRole?: 'organizer' | 'attendee'
+  eventRole?: 'organizer' | 'attendee' | 'watcher'
   timeDisplayMode?: 'clock' | 'relative'
   dateTime?: string
 }>()
@@ -152,6 +152,12 @@ function onTouchEnd() {
   color: var(--color-text-bright);
 }
 
+.event-card__badge--watcher {
+  background: var(--color-glass);
+  color: var(--color-text-secondary);
+  border: 1px solid var(--color-glass-border);
+}
+
 .event-card__delete {
   flex-shrink: 0;
   width: 28px;