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

CLAUDE.md

@@ -39,6 +39,8 @@ npm run start -w @caldav/server        # Run compiled server (port 3000)
 | | Expo | Development platform |
 | | Expo-Router | File-based routing |
 | | NativeWind | Tailwind CSS for React Native |
+| | Zustand | State management |
+| | FlashList | High-performance lists |
 | Backend | Express.js | Web framework |
 | | MongoDB | Database |
 | | Mongoose | ODM |
@@ -57,13 +59,39 @@ packages/shared  - @caldav/shared - Shared TypeScript types and models
 
 ### Frontend Architecture (apps/client)
 
-**Screens** (`src/app/`): Login, Register, Calendar, Chat, Event Detail, Notes - file-based routing via Expo-Router
+```
+src/
+├── app/                        # Expo-Router file-based routing
+│   ├── _layout.tsx             # Root Stack layout
+│   ├── index.tsx               # Entry redirect
+│   ├── login.tsx               # Login screen
+│   ├── register.tsx            # Registration screen
+│   ├── (tabs)/                 # Tab navigation group
+│   │   ├── _layout.tsx         # Tab bar configuration
+│   │   ├── chat.tsx            # Chat screen (AI conversation)
+│   │   └── calendar.tsx        # Calendar overview
+│   ├── event/
+│   │   └── [id].tsx            # Event detail screen (dynamic route)
+│   └── note/
+│       └── [id].tsx            # Note editor for event (dynamic route)
+├── components/
+│   ├── BaseBackground.tsx      # Common screen wrapper
+│   ├── Header.tsx              # Header component
+│   ├── EventCard.tsx           # Event card for calendar display
+│   └── EventConfirmDialog.tsx  # AI-proposed event confirmation modal
+├── services/
+│   ├── index.ts                # Re-exports all services
+│   ├── ApiClient.ts            # HTTP client (get, post, put, delete)
+│   ├── AuthService.ts          # login(), register(), logout(), refresh()
+│   ├── EventService.ts         # getAll(), getById(), getByDateRange(), create(), update(), delete()
+│   └── ChatService.ts          # sendMessage(), confirmEvent(), rejectEvent(), getConversations(), getConversation()
+└── stores/                     # Zustand state management
+    ├── index.ts                # Re-exports all stores
+    ├── AuthStore.ts            # user, token, isAuthenticated, login(), logout(), setToken()
+    └── EventsStore.ts          # events[], setEvents(), addEvent(), updateEvent(), deleteEvent()
+```
 
-**Services**: API Client for HTTP requests, plus AuthService, EventService, ChatService for domain logic
-
-**Components**: Reusable UI (EventCard, ChatBubble, EventConfirmDialog, MonthSelector)
-
-**Stores**: AuthStore (user + token), EventsStore (calendar events)
+**Routing:** Tab-based navigation with Chat and Calendar as main screens. Auth screens (login, register) outside tabs. Dynamic routes for event detail and note editing.
 
 ### Backend Architecture (apps/server)
 
@@ -176,7 +204,14 @@ The repository pattern allows swapping databases:
 
 **Shared:** Types and DTOs defined and exported.
 
-**Frontend:** Calendar screen partially implemented, Chat screen exists but not connected to backend.
+**Frontend:** Skeleton complete with file-based routing structure:
+- Tab navigation (Chat, Calendar) implemented with basic UI
+- Calendar screen has month navigation and grid display (partially functional)
+- Chat screen has message list UI with FlashList (mock data only)
+- Auth screens (Login, Register), Event Detail, and Note screens exist as skeletons
+- Services (ApiClient, AuthService, EventService, ChatService) defined with `throw new Error('Not implemented')`
+- Zustand stores (AuthStore, EventsStore) defined with `throw new Error('Not implemented')`
+- Components (EventCard, EventConfirmDialog) exist as skeletons
 
 ## Documentation