Fix TypeScript type errors in frontend test files

- Add missing timezone field to CreateEventResponse mock
- Fix createTestRouter signature to accept optional token parameter
- Add non-null assertion for dateField element access

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/views/__tests__/EventCreateView.spec.ts

@@ -173,6 +173,7 @@ describe('EventCreateView', () => {
         organizerToken: 'org-456',
         title: 'Birthday Party',
         dateTime: '2026-12-25T18:00:00+01:00',
+        timezone: 'Europe/Berlin',
         expiryDate: '2026-12-24',
       },
       error: undefined,