feat: Integrate Expo mobile application into monorepo (#16)

- Add mobile/ directory with Expo React Native project
- Create API client with JWT auth and token refresh using AsyncStorage
- Implement AuthContext for login/register/logout flow
- Add screens: Login, Register, Library, Search, Settings
- Set up React Navigation with AuthStack and MainTabs
- Create packages/shared/ with shared types and utilities
- Add shared validation utilities (email, password strength)
- Update root package.json workspaces to include mobile + shared
- Add spec document docs/backend/009-expo-integration.md

README.md

@@ -14,7 +14,7 @@ cloud-reader/
 │   │   └── annotations/ # Bookmarks and notes
 │   ├── manage.py
 │   └── requirements.txt
-├── frontend/        # React + Vite + TypeScript (canonical frontend)
+├── frontend/        # React + Vite + TypeScript (web frontend)
 │   ├── src/
 │   │   ├── api/         # API client (axios with JWT refresh)
 │   │   ├── components/  # Reusable components
@@ -23,6 +23,23 @@ cloud-reader/
 │   │   ├── pages/       # Route pages (Library, Reader, AddBook, Auth, Settings)
 │   │   └── types/       # TypeScript type definitions
 │   └── package.json
+├── mobile/          # Expo React Native app (mobile frontend)
+│   ├── src/
+│   │   ├── api/         # API client (axios with JWT refresh via AsyncStorage)
+│   │   ├── components/  # Reusable UI components
+│   │   ├── context/     # Auth context
+│   │   ├── hooks/       # Custom hooks
+│   │   ├── navigation/  # React Navigation (Auth stack + Main tabs)
+│   │   ├── screens/     # Screen-level components (Login, Library, etc.)
+│   │   └── types/       # Mobile-specific types
+│   ├── App.tsx
+│   └── app.json
+├── packages/
+│   └── shared/       # @cloud-reader/shared — domain types & utilities
+│       └── src/
+│           ├── types.ts   # Shared domain types (Book, User, Bookmark, Note, etc.)
+│           └── utils.ts   # Date formatting, validation, API endpoint constants
+├── package.json      # Root — yarn workspaces config
 └── docker-compose.yml
 ```
 
@@ -50,8 +67,24 @@ yarn install
 yarn dev
 ```
 
+### Mobile (Expo)
+```bash
+# From monorepo root — installs all workspaces including mobile
+yarn install
+
+# Start Expo dev server
+yarn workspace @cloud-reader/mobile start
+
+# Or cd into mobile and run directly
+cd mobile
+npx expo start
+```
+
+> The mobile app requires the backend to be running. Set `EXPO_PUBLIC_API_URL` environment variable in your shell or `.env` file to point to the backend (defaults to `http://10.0.2.2:8000` for Android emulator).
+
 ## Migration Notes
 Consolidated from duplicate `api/` + `web/` into single `backend/` + `frontend/` canonical structure.
 - `backend/` kept as canonical; `api/` features (e-book uploads, reading progress, reading settings) merged in.
 - `frontend/` kept as canonical; `web/` pages (Library, Reader, AddBook, Auth, Settings) merged in.
-- `api/` and `web/` directories removed.
+- `api/` and `web/` directories removed.
+- `mobile/` added as Expo React Native app with shared `@cloud-reader/shared` package.