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/app.json

@@ -0,0 +1,28 @@
+{
+  "expo": {
+    "name": "Cloud Reader",
+    "slug": "cloud-reader",
+    "version": "1.0.0",
+    "orientation": "portrait",
+    "icon": "./assets/icon.png",
+    "userInterfaceStyle": "automatic",
+    "newArchEnabled": true,
+    "splash": {
+      "backgroundColor": "#1a1a2e"
+    },
+    "ios": {
+      "supportsTablet": true,
+      "bundleIdentifier": "com.cloudreader.app"
+    },
+    "android": {
+      "adaptiveIcon": {
+        "backgroundColor": "#1a1a2e"
+      },
+      "package": "com.cloudreader.app"
+    },
+    "plugins": [
+      "expo-document-picker",
+      "expo-file-system"
+    ]
+  }
+}