added guardian letters with chat and multiple answer functionalities

lib/api/marianumconnect/errors/marianumconnect_error.dart

@@ -6,6 +6,22 @@ import '../../errors/network_exception.dart';
 import '../../errors/parse_exception.dart';
 import '../../errors/server_exception.dart';
 
+/// The error code of a rejected call: the server answers either with JSON
+/// `{"error": "<code>"}` or with the plain text `Fehler: <code>` of its
+/// generic error handler.
+String? marianumConnectErrorCode(Object? body) {
+  if (body is Map) {
+    final code = body['error'];
+    return code is String ? code : null;
+  }
+  if (body is String) {
+    const prefix = 'Fehler: ';
+    return (body.startsWith(prefix) ? body.substring(prefix.length) : body)
+        .trim();
+  }
+  return null;
+}
+
 /// Converts a DioException raised against the Marianum-Connect API into one of
 /// the app's typed AppExceptions. Keeps the dio dependency out of call sites
 /// that just want to render an error message.