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/__tests__/EventCard.spec.ts

@@ -73,4 +73,28 @@ describe('EventCard', () => {
     await wrapper.find('.event-card__delete').trigger('click')
     expect(wrapper.emitted('delete')).toEqual([['abc-123']])
   })
+
+  it('displays clock time when timeDisplayMode is clock', () => {
+    const wrapper = mountCard({
+      timeDisplayMode: 'clock',
+      dateTime: '2026-03-11T18:30:00',
+    })
+    const timeText = wrapper.find('.event-card__time').text()
+    // Locale-dependent: could be "18:30" or "06:30 PM"
+    expect(timeText).toMatch(/(?:18.30|6.30\s*PM)/i)
+  })
+
+  it('displays relative time when timeDisplayMode is relative', () => {
+    const wrapper = mountCard({
+      relativeTime: '3 days ago',
+      timeDisplayMode: 'relative',
+      dateTime: '2026-03-08T10:00:00',
+    })
+    expect(wrapper.find('.event-card__time').text()).toBe('3 days ago')
+  })
+
+  it('falls back to relativeTime when timeDisplayMode is not set', () => {
+    const wrapper = mountCard({ relativeTime: 'in 3 days' })
+    expect(wrapper.find('.event-card__time').text()).toBe('in 3 days')
+  })
 })