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__/DateSubheader.spec.ts

@@ -0,0 +1,17 @@
+import { describe, it, expect } from 'vitest'
+import { mount } from '@vue/test-utils'
+import DateSubheader from '../DateSubheader.vue'
+
+describe('DateSubheader', () => {
+  it('renders the date label as an h3', () => {
+    const wrapper = mount(DateSubheader, { props: { label: 'Wed, 12 Mar' } })
+    const h3 = wrapper.find('h3')
+    expect(h3.exists()).toBe(true)
+    expect(h3.text()).toBe('Wed, 12 Mar')
+  })
+
+  it('applies the date-subheader class', () => {
+    const wrapper = mount(DateSubheader, { props: { label: 'Fri, 14 Mar' } })
+    expect(wrapper.find('.date-subheader').exists()).toBe(true)
+  })
+})