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/stores/AuthStore.ts

@@ -0,0 +1,26 @@
+import { create } from 'zustand';
+import { User } from '@caldav/shared';
+
+interface AuthState {
+  user: User | null;
+  token: string | null;
+  isAuthenticated: boolean;
+  login: (user: User, token: string) => void;
+  logout: () => void;
+  setToken: (token: string) => void;
+}
+
+export const useAuthStore = create<AuthState>((set) => ({
+  user: null,
+  token: null,
+  isAuthenticated: false,
+  login: (_user: User, _token: string) => {
+    throw new Error('Not implemented');
+  },
+  logout: () => {
+    throw new Error('Not implemented');
+  },
+  setToken: (_token: string) => {
+    throw new Error('Not implemented');
+  },
+}));