added guardian letters with chat and multiple answer functionalities

lib/api/marianumconnect/marianumconnect_query.dart

@@ -1,5 +1,8 @@
+import 'dart:typed_data';
+
 import 'package:dio/dio.dart';
 
+import '../errors/app_exception.dart';
 import 'errors/marianumconnect_error.dart';
 import 'marianumconnect_api.dart';
 import 'marianumconnect_endpoint.dart';
@@ -23,10 +26,14 @@ abstract class MarianumConnectQuery {
     try {
       return await body();
     } on DioException catch (e) {
-      throw mapMarianumConnectError(e);
+      throw mapError(e);
     }
   }
 
+  /// The AppException a failed call surfaces as. Query families with domain
+  /// error codes override this instead of re-implementing [guard].
+  AppException mapError(DioException error) => mapMarianumConnectError(error);
+
   /// GETs [path] and parses the JSON object body with [fromJson].
   Future<T> getObject<T>(
     String path,
@@ -55,6 +62,16 @@ abstract class MarianumConnectQuery {
         .toList();
   });
 
+  /// GETs the raw bytes of [path] (files that need the bearer token).
+  Future<Uint8List> getBytes(String path) => guard(() async {
+    final response = await dio.get<List<int>>(
+      endpoint(path),
+      options: Options(responseType: ResponseType.bytes),
+    );
+    final bytes = response.data!;
+    return bytes is Uint8List ? bytes : Uint8List.fromList(bytes);
+  });
+
   /// Formats [d] as an ISO `yyyy-MM-dd` day for MarianumConnect query params.
   String isoDate(DateTime d) =>
       '${d.year.toString().padLeft(4, '0')}-${d.month.toString().padLeft(2, '0')}-${d.day.toString().padLeft(2, '0')}';