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/AuthService.ts

@@ -0,0 +1,19 @@
+import { LoginDTO, CreateUserDTO, AuthResponse } from '@caldav/shared';
+
+export const AuthService = {
+  login: async (_credentials: LoginDTO): Promise<AuthResponse> => {
+    throw new Error('Not implemented');
+  },
+
+  register: async (_data: CreateUserDTO): Promise<AuthResponse> => {
+    throw new Error('Not implemented');
+  },
+
+  logout: async (): Promise<void> => {
+    throw new Error('Not implemented');
+  },
+
+  refresh: async (): Promise<AuthResponse> => {
+    throw new Error('Not implemented');
+  },
+};