Stabilize frontend login. Make chat messages multiline.

frontend/lib/user_checker.dart

@@ -44,12 +44,18 @@ class _UserCheckerState extends State<UserChecker> {
   }
 
   void _handleLogin(String user, String password) async {
+    bool success = await Backend.auth(user, password);
+    if (!success) {
+      setState(() {
+        isLoading = false;
+      });
+      return;
+    }
+  
     setState(() {
       username = user;
       isLoading = true;
     });
-  
-    await Backend.auth(user, password);
     await _storage.write(key: 'username', value: user);
     await _storage.write(key: 'password', value: password);