simplified code across the app and consolidated duplicated helpers

lib/api/marianumcloud/login_flow/login_flow_api.dart

@@ -29,12 +29,7 @@ class LoginFlowApi {
         headers: {'Accept': 'application/json', 'User-Agent': userAgent},
       ),
     ))!;
-    if (response.statusCode < 200 || response.statusCode >= 300) {
-      throwForStatus(
-        response.statusCode,
-        httpErrorDetail(label, response.body, response.statusCode),
-      );
-    }
+    ensureSuccess(label, response);
     return LoginFlowInit.fromJson(
       jsonDecode(utf8.decode(response.bodyBytes)) as Map<String, dynamic>,
     );
@@ -53,12 +48,7 @@ class LoginFlowApi {
       ),
     ))!;
     if (response.statusCode == 404) return null;
-    if (response.statusCode < 200 || response.statusCode >= 300) {
-      throwForStatus(
-        response.statusCode,
-        httpErrorDetail(label, response.body, response.statusCode),
-      );
-    }
+    ensureSuccess(label, response);
     return LoginFlowCredentials.fromJson(
       jsonDecode(utf8.decode(response.bodyBytes)) as Map<String, dynamic>,
     );