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

package.json

@@ -4,6 +4,15 @@
   "private": true,
   "workspaces": [
     "frontend",
-    "backend"
-  ]
+    "backend",
+    "mobile",
+    "packages/shared"
+  ],
+  "scripts": {
+    "dev:frontend": "yarn workspace @cloud-reader/frontend dev",
+    "dev:backend": "cd backend && python manage.py runserver",
+    "start:mobile": "yarn workspace @cloud-reader/mobile start",
+    "build:shared": "yarn workspace @cloud-reader/shared build",
+    "install:all": "yarn install"
+  }
 }