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>

CLAUDE.md

@@ -53,6 +53,8 @@ The following skills are available and should be used for their respective purpo
 ## Active Technologies
 - Java 25 (backend), TypeScript 5.9 (frontend) + Spring Boot 3.5.x, Vue 3, Vue Router 5, openapi-fetch, openapi-typescript (007-view-event)
 - PostgreSQL (JPA via Spring Data, Liquibase migrations) (007-view-event)
+- TypeScript 5.9 (frontend only) + Vue 3, Vue Router 5 (existing — no additions) (010-event-list-grouping)
+- localStorage via `useEventStorage.ts` composable (existing — no changes) (010-event-list-grouping)
 
 ## Recent Changes
 - 007-view-event: Added Java 25 (backend), TypeScript 5.9 (frontend) + Spring Boot 3.5.x, Vue 3, Vue Router 5, openapi-fetch, openapi-typescript