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/ChatService.ts

@@ -0,0 +1,39 @@
+import {
+  SendMessageDTO,
+  ChatResponse,
+  ChatMessage,
+  ConversationSummary,
+  GetMessagesOptions,
+  CalendarEvent,
+} from '@caldav/shared';
+
+export const ChatService = {
+  sendMessage: async (_data: SendMessageDTO): Promise<ChatResponse> => {
+    throw new Error('Not implemented');
+  },
+
+  confirmEvent: async (
+    _conversationId: string,
+    _messageId: string
+  ): Promise<CalendarEvent> => {
+    throw new Error('Not implemented');
+  },
+
+  rejectEvent: async (
+    _conversationId: string,
+    _messageId: string
+  ): Promise<void> => {
+    throw new Error('Not implemented');
+  },
+
+  getConversations: async (): Promise<ConversationSummary[]> => {
+    throw new Error('Not implemented');
+  },
+
+  getConversation: async (
+    _id: string,
+    _options?: GetMessagesOptions
+  ): Promise<ChatMessage[]> => {
+    throw new Error('Not implemented');
+  },
+};