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/DateSubheader.vue

@@ -0,0 +1,19 @@
+<template>
+  <h3 class="date-subheader">{{ label }}</h3>
+</template>
+
+<script setup lang="ts">
+defineProps<{
+  label: string
+}>()
+</script>
+
+<style scoped>
+.date-subheader {
+  font-size: 0.85rem;
+  font-weight: 500;
+  color: rgba(255, 255, 255, 0.85);
+  margin: 0;
+  padding: var(--spacing-xs) 0;
+}
+</style>