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

lib/api/demo/demo_marianumconnect.dart

@@ -1,3 +1,4 @@
+import 'data/demo_absence.dart';
 import 'data/demo_breaker.dart';
 import 'data/demo_holidays.dart';
 import 'data/demo_timetable.dart';
@@ -39,6 +40,10 @@ class DemoMarianumConnect {
             .toList();
       case 'breaker':
         return DemoBreaker.none().toJson();
+      case 'absence/classes':
+        return DemoAbsence.classes();
+      case 'absence/prefill':
+        return DemoAbsence.prefill().toJson();
       case 'timetable/elements/teachers':
       case 'timetable/elements/students':
       case 'timetable/elements/classes':