implemented absence report module with form validation, prefill support, and API integration

lib/api/marianumconnect/queries/absence/absence_classes.dart

@@ -0,0 +1,13 @@
+import '../../marianumconnect_query.dart';
+
+/// GETs the selectable classes for the absence form (`absence/classes`). The
+/// body is a bare JSON string array, so [getList] (which maps objects) does not
+/// fit — read the raw list and cast.
+class AbsenceClasses extends MarianumConnectQuery {
+  AbsenceClasses({super.dio});
+
+  Future<List<String>> run() => guard(() async {
+    final response = await dio.get<List<dynamic>>(endpoint('absence/classes'));
+    return response.data!.cast<String>();
+  });
+}