simplify: dedupe boilerplate and remove dead code across all layers

lib/api/marianumcloud/nextcloud_ocs.dart

@@ -1,3 +1,5 @@
+import 'dart:convert';
+
 import '../../model/account_data.dart';
 import '../../model/endpoint_data.dart';
 
@@ -6,6 +8,11 @@ import '../../model/endpoint_data.dart';
 class NextcloudOcs {
   NextcloudOcs._();
 
+  /// Decodes an OCS v2 JSON envelope and returns its `ocs` object (the wrapper
+  /// every response nests its `meta`/`data` under).
+  static Map<String, dynamic> decode(String raw) =>
+      (jsonDecode(raw) as Map<String, dynamic>)['ocs'] as Map<String, dynamic>;
+
   static Map<String, String> headers() => {
     'Accept': 'application/json',
     'OCS-APIRequest': 'true',