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__/SectionHeader.spec.ts

@@ -0,0 +1,27 @@
+import { describe, it, expect } from 'vitest'
+import { mount } from '@vue/test-utils'
+import SectionHeader from '../SectionHeader.vue'
+
+describe('SectionHeader', () => {
+  it('renders the section label as an h2', () => {
+    const wrapper = mount(SectionHeader, { props: { label: 'Today' } })
+    const h2 = wrapper.find('h2')
+    expect(h2.exists()).toBe(true)
+    expect(h2.text()).toBe('Today')
+  })
+
+  it('does not apply emphasized class by default', () => {
+    const wrapper = mount(SectionHeader, { props: { label: 'Later' } })
+    expect(wrapper.find('.section-header--emphasized').exists()).toBe(false)
+  })
+
+  it('applies emphasized class when emphasized prop is true', () => {
+    const wrapper = mount(SectionHeader, { props: { label: 'Today', emphasized: true } })
+    expect(wrapper.find('.section-header--emphasized').exists()).toBe(true)
+  })
+
+  it('does not apply emphasized class when emphasized prop is false', () => {
+    const wrapper = mount(SectionHeader, { props: { label: 'Past', emphasized: false } })
+    expect(wrapper.find('.section-header--emphasized').exists()).toBe(false)
+  })
+})