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

packages/shared/package.json

@@ -0,0 +1,14 @@
+{
+  "name": "@cloud-reader/shared",
+  "version": "1.0.0",
+  "private": true,
+  "main": "src/index.ts",
+  "types": "src/index.ts",
+  "scripts": {
+    "build": "tsc",
+    "typecheck": "tsc --noEmit"
+  },
+  "dependencies": {
+    "typescript": "~5.7.0"
+  }
+}