Add temporal grouping to event list (Today/This Week/Next Week/Later/Past)

Group events into five temporal sections with section headers, date subheaders,
and context-aware time display (clock time for upcoming, relative for past).
Includes new useEventGrouping composable, SectionHeader and DateSubheader
components, full unit and E2E test coverage.

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

frontend/src/components/EventCard.vue

@@ -9,7 +9,7 @@
   >
     <RouterLink :to="`/events/${eventToken}`" class="event-card__link">
       <span class="event-card__title">{{ title }}</span>
-      <span class="event-card__time">{{ relativeTime }}</span>
+      <span class="event-card__time">{{ displayTime }}</span>
     </RouterLink>
     <span v-if="eventRole" class="event-card__badge" :class="`event-card__badge--${eventRole}`">
       {{ eventRole === 'organizer' ? 'Organizer' : 'Attendee' }}
@@ -35,12 +35,21 @@ const props = defineProps<{
   relativeTime: string
   isPast: boolean
   eventRole?: 'organizer' | 'attendee'
+  timeDisplayMode?: 'clock' | 'relative'
+  dateTime?: string
 }>()
 
 const emit = defineEmits<{
   delete: [eventToken: string]
 }>()
 
+const displayTime = computed(() => {
+  if (props.timeDisplayMode === 'clock' && props.dateTime) {
+    return new Intl.DateTimeFormat(undefined, { hour: '2-digit', minute: '2-digit' }).format(new Date(props.dateTime))
+  }
+  return props.relativeTime
+})
+
 const SWIPE_THRESHOLD = 80
 
 const startX = ref(0)