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/SectionHeader.vue

@@ -0,0 +1,27 @@
+<template>
+  <h2 class="section-header" :class="{ 'section-header--emphasized': emphasized }">
+    {{ label }}
+  </h2>
+</template>
+
+<script setup lang="ts">
+defineProps<{
+  label: string
+  emphasized?: boolean
+}>()
+</script>
+
+<style scoped>
+.section-header {
+  font-size: 1rem;
+  font-weight: 700;
+  color: var(--color-text);
+  margin: 0;
+  padding: var(--spacing-sm) 0;
+}
+
+.section-header--emphasized {
+  font-size: 1.1rem;
+  font-weight: 800;
+}
+</style>