feat: Implement subscription management and article saving features

- Added subscriptionPlan and subscriptionExpiresAt fields to UserProfile model.
- Enhanced profile routes to handle subscription state updates and expiration logic.
- Introduced email and password validation during user registration.
- Implemented link stripping for processed articles in the articles route.
- Added save article functionality with appropriate UI feedback in the frontend.
- Updated AccountSettings and FeedContent components to reflect subscription state and saved articles.
- Improved error handling and local storage management for user preferences.

frontend-BriefAI/src/pages/RegisterPage.tsx

@@ -39,6 +39,13 @@ function RegisterPage({ onRegisterSuccess }: RegisterPageProps) {
       // ignore parsing errors
     }
 
+    // Basic email format validation on client
+    const emailRegex = /^[^\s@]+@[^\s@]+\.[^\s@]+$/
+    if (!emailRegex.test(String(email).toLowerCase())) {
+      setError('Formato email non valido.')
+      return
+    }
+
     try {
       await register({
         email,