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

mobile/package.json

@@ -0,0 +1,35 @@
+{
+  "name": "@cloud-reader/mobile",
+  "version": "1.0.0",
+  "private": true,
+  "main": "expo/AppEntry.js",
+  "scripts": {
+    "start": "expo start",
+    "android": "expo start --android",
+    "ios": "expo start --ios",
+    "web": "expo start --web",
+    "lint": "eslint ."
+  },
+  "dependencies": {
+    "expo": "~52.0.0",
+    "expo-status-bar": "~2.0.0",
+    "react": "^19.0.0",
+    "react-native": "0.76.6",
+    "react-native-safe-area-context": "4.14.1",
+    "react-native-screens": "~4.4.0",
+    "@react-navigation/native": "^7.0.0",
+    "@react-navigation/native-stack": "^7.0.0",
+    "@react-navigation/bottom-tabs": "^7.0.0",
+    "axios": "^1.7.9",
+    "@react-native-async-storage/async-storage": "2.1.0",
+    "expo-document-picker": "~13.0.0",
+    "expo-file-system": "~18.0.0",
+    "react-native-gesture-handler": "~2.20.0",
+    "react-native-reanimated": "~3.16.0",
+    "@cloud-reader/shared": "*"
+  },
+  "devDependencies": {
+    "@types/react": "~19.0.0",
+    "typescript": "~5.7.0"
+  }
+}