add MarianumConnectQuery base and adopt it in list queries

lib/api/marianumconnect/marianumconnect_query.dart

@@ -0,0 +1,29 @@
+import 'package:dio/dio.dart';
+
+import 'errors/marianumconnect_error.dart';
+import 'marianumconnect_api.dart';
+import 'marianumconnect_endpoint.dart';
+
+/// Shared base for MarianumConnect API queries. Owns the [dio] client (the
+/// shared authenticated singleton by default) and routes calls through [guard]
+/// so every query maps a DioException to the app's typed AppExceptions the same
+/// way instead of repeating the try/catch. Subclasses with bespoke error or
+/// lifecycle handling (own dio, silent failure, custom status mapping) may skip
+/// [guard] and still reuse [dio]/[endpoint].
+abstract class MarianumConnectQuery {
+  final Dio dio;
+
+  MarianumConnectQuery({Dio? dio}) : dio = dio ?? MarianumConnectApi.dio();
+
+  /// Resolves [path] against the active mobile-API base URL.
+  String endpoint(String path) => MarianumConnectEndpoint.resolve(path);
+
+  /// Runs [body], converting any DioException into the matching AppException.
+  Future<T> guard<T>(Future<T> Function() body) async {
+    try {
+      return await body();
+    } on DioException catch (e) {
+      throw mapMarianumConnectError(e);
+    }
+  }
+}