loading state and error handling refactor

lib/api/errors/server_exception.dart

@@ -0,0 +1,14 @@
+import 'app_exception.dart';
+
+class ServerException extends AppException {
+  final int statusCode;
+
+  ServerException({
+    required this.statusCode,
+    String? userMessage,
+    super.technicalDetails,
+  }) : super(
+          userMessage: userMessage ?? 'Der Server hat gerade Probleme (Status $statusCode). Bitte später erneut versuchen.',
+          allowRetry: true,
+        );
+}