implement frontend skeleton with tab navigation and service layer

- Add tab-based navigation (Chat, Calendar) using Expo-Router
- Create auth screens (login, register) as skeletons
- Add dynamic routes for event detail and note editing
- Implement service layer (ApiClient, AuthService, EventService, ChatService)
- Add Zustand stores (AuthStore, EventsStore) for state management
- Create EventCard and EventConfirmDialog components
- Update CLAUDE.md with new frontend architecture documentation
- Add Zustand and FlashList to technology stack

apps/client/src/services/index.ts

@@ -0,0 +1,4 @@
+export { ApiClient, API_BASE_URL } from './ApiClient';
+export { AuthService } from './AuthService';
+export { EventService } from './EventService';
+export { ChatService } from './ChatService';