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/EventList.vue

@@ -1,15 +1,30 @@
 <template>
-  <div class="event-list" role="list" aria-label="Your events">
-    <div v-for="event in sortedEvents" :key="event.eventToken" role="listitem">
-      <EventCard
-        :event-token="event.eventToken"
-        :title="event.title"
-        :relative-time="formatRelativeTime(event.dateTime)"
-        :is-past="isPast(event.dateTime)"
-        :event-role="getRole(event)"
-        @delete="requestDelete"
-      />
-    </div>
+  <div class="event-list">
+    <section
+      v-for="section in groupedSections"
+      :key="section.key"
+      :aria-label="section.label"
+      class="event-section"
+    >
+      <SectionHeader :label="section.label" :emphasized="section.emphasized" />
+      <div role="list">
+        <template v-for="group in section.dateGroups" :key="group.dateKey">
+          <DateSubheader v-if="group.showSubheader" :label="group.label" />
+          <div v-for="event in group.events" :key="event.eventToken" role="listitem">
+            <EventCard
+              :event-token="event.eventToken"
+              :title="event.title"
+              :relative-time="formatRelativeTime(event.dateTime)"
+              :is-past="section.key === 'past'"
+              :event-role="getRole(event)"
+              :time-display-mode="section.key === 'past' ? 'relative' : 'clock'"
+              :date-time="event.dateTime"
+              @delete="requestDelete"
+            />
+          </div>
+        </template>
+      </div>
+    </section>
     <ConfirmDialog
       :open="!!pendingDeleteToken"
       title="Remove event?"
@@ -25,8 +40,11 @@
 <script setup lang="ts">
 import { computed, ref } from 'vue'
 import { useEventStorage, isValidStoredEvent } from '../composables/useEventStorage'
+import { useEventGrouping } from '../composables/useEventGrouping'
 import { formatRelativeTime } from '../composables/useRelativeTime'
 import EventCard from './EventCard.vue'
+import SectionHeader from './SectionHeader.vue'
+import DateSubheader from './DateSubheader.vue'
 import ConfirmDialog from './ConfirmDialog.vue'
 import type { StoredEvent } from '../composables/useEventStorage'
 
@@ -49,29 +67,26 @@ function cancelDelete() {
   pendingDeleteToken.value = null
 }
 
-function isPast(dateTime: string): boolean {
-  return new Date(dateTime) < new Date()
-}
-
 function getRole(event: StoredEvent): 'organizer' | 'attendee' | undefined {
   if (event.organizerToken) return 'organizer'
   if (event.rsvpToken) return 'attendee'
   return undefined
 }
 
-const sortedEvents = computed(() => {
+const groupedSections = computed(() => {
   const valid = getStoredEvents().filter(isValidStoredEvent)
-  const now = new Date()
-  const upcoming = valid.filter((e) => new Date(e.dateTime) >= now)
-  const past = valid.filter((e) => new Date(e.dateTime) < now)
-  upcoming.sort((a, b) => new Date(a.dateTime).getTime() - new Date(b.dateTime).getTime())
-  past.sort((a, b) => new Date(b.dateTime).getTime() - new Date(a.dateTime).getTime())
-  return [...upcoming, ...past]
+  return useEventGrouping(valid)
 })
 </script>
 
 <style scoped>
 .event-list {
+  display: flex;
+  flex-direction: column;
+  gap: var(--spacing-md);
+}
+
+.event-section [role="list"] {
   display: flex;
   flex-direction: column;
   gap: var(--spacing-sm);