simplified code across the app and consolidated duplicated helpers

lib/view/login/widgets/login_card.dart

@@ -32,25 +32,14 @@ class _LoginCardState extends State<LoginCard> {
   final _passwordController = TextEditingController();
   final _passwordFocus = FocusNode();
 
-  @override
-  void initState() {
-    super.initState();
-    widget.controller.addListener(_onControllerChange);
-  }
-
   @override
   void dispose() {
-    widget.controller.removeListener(_onControllerChange);
     _usernameController.dispose();
     _passwordController.dispose();
     _passwordFocus.dispose();
     super.dispose();
   }
 
-  void _onControllerChange() {
-    if (mounted) setState(() {});
-  }
-
   String? _required(String? value) =>
       (value ?? '').trim().isEmpty ? 'Eingabe erforderlich' : null;
 
@@ -81,7 +70,10 @@ class _LoginCardState extends State<LoginCard> {
   }
 
   @override
-  Widget build(BuildContext context) {
+  Widget build(BuildContext context) =>
+      ListenableBuilder(listenable: widget.controller, builder: _buildForm);
+
+  Widget _buildForm(BuildContext context, Widget? _) {
     final theme = Theme.of(context);
     final loading = widget.controller.loading;
     return Form(