loading state and error handling refactor

lib/main.dart

@@ -32,7 +32,7 @@ import 'storage/settings.dart';
 import 'theming/dark_app_theme.dart';
 import 'theming/light_app_theme.dart';
 import 'view/login/login.dart';
-import 'widget/placeholder_view.dart';
+import 'widget/app_progress_indicator.dart';
 
 Future<void> main() async {
   log('MarianumMobile started');
@@ -68,9 +68,21 @@ Future<void> main() async {
   );
 
   if (kReleaseMode) {
-    ErrorWidget.builder = (error) => PlaceholderView(
-          icon: Icons.phonelink_erase_rounded,
-          text: error.toStringShort(),
+    ErrorWidget.builder = (error) => Material(
+          color: Colors.white,
+          child: Center(
+            child: Padding(
+              padding: const EdgeInsets.all(24),
+              child: Column(
+                mainAxisSize: MainAxisSize.min,
+                children: [
+                  const Icon(Icons.phonelink_erase_rounded, size: 40),
+                  const SizedBox(height: 12),
+                  Text(error.toStringShort(), textAlign: TextAlign.center),
+                ],
+              ),
+            ),
+          ),
         );
   }
 
@@ -156,7 +168,18 @@ class _MainState extends State<Main> {
                         case AccountStatus.loggedOut:
                           return const Login();
                         case AccountStatus.undefined:
-                          return const PlaceholderView(icon: Icons.timer, text: 'Daten werden geladen');
+                          return const Scaffold(
+                            body: Center(
+                              child: Column(
+                                mainAxisAlignment: MainAxisAlignment.center,
+                                children: [
+                                  AppProgressIndicator.large(),
+                                  SizedBox(height: 16),
+                                  Text('Konto wird geladen…'),
+                                ],
+                              ),
+                            ),
+                          );
                       }
                     },
                   ),