simplify: dedupe boilerplate and remove dead code across all layers

lib/api/marianumconnect/marianumconnect_query.dart

@@ -26,4 +26,36 @@ abstract class MarianumConnectQuery {
       throw mapMarianumConnectError(e);
     }
   }
+
+  /// GETs [path] and parses the JSON object body with [fromJson].
+  Future<T> getObject<T>(
+    String path,
+    T Function(Map<String, dynamic> json) fromJson, {
+    Map<String, dynamic>? queryParameters,
+  }) => guard(() async {
+    final response = await dio.get<Map<String, dynamic>>(
+      endpoint(path),
+      queryParameters: queryParameters,
+    );
+    return fromJson(response.data!);
+  });
+
+  /// GETs [path] and maps each element of the JSON array body with [fromJson].
+  Future<List<T>> getList<T>(
+    String path,
+    T Function(Map<String, dynamic> json) fromJson, {
+    Map<String, dynamic>? queryParameters,
+  }) => guard(() async {
+    final response = await dio.get<List<dynamic>>(
+      endpoint(path),
+      queryParameters: queryParameters,
+    );
+    return response.data!
+        .map((e) => fromJson(e as Map<String, dynamic>))
+        .toList();
+  });
+
+  /// 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')}';
 }