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__/EventDetailView.spec.ts

@@ -10,7 +10,7 @@ vi.mock('@/api/client', () => ({
   },
 }))
 
-function createTestRouter() {
+function createTestRouter(_token?: string) {
   return createRouter({
     history: createMemoryHistory(),
     routes: [
@@ -84,7 +84,7 @@ describe('EventDetailView', () => {
     const wrapper = await mountWithToken()
     await flushPromises()
 
-    const dateField = wrapper.findAll('.detail__value')[0]
+    const dateField = wrapper.findAll('.detail__value')[0]!
     expect(dateField.text()).toContain('(Europe/Berlin)')
     // The formatted date part is locale-dependent but should contain the year
     expect(dateField.text()).toContain('2026')