Auth: Save user ID instead of username in auth session

src/Frontend/Login/LoginController.php

@@ -66,7 +66,7 @@ class LoginController extends BaseController
         }
 
         // Set login session
-        $_SESSION['username'] = $user->getUsername();
+        $_SESSION['user_id'] = $user->getId();
         return $response
             ->withHeader('Location', '/')
             ->withStatus(303);