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

lib/api/demo/data/demo_absence.dart

@@ -0,0 +1,17 @@
+import '../../marianumconnect/queries/absence/absence_prefill_response.dart';
+
+/// Demo fixtures for the absence-report form: the class dropdown and the
+/// identity/phone prefill. Typed (compile-checked) so a field rename can't
+/// silently drift the demo shape away from what the queries parse.
+class DemoAbsence {
+  const DemoAbsence._();
+
+  static List<String> classes() => const ['5a', '6b', '7c', '9d', '10a', 'Q1'];
+
+  static AbsencePrefillResponse prefill() => AbsencePrefillResponse(
+    firstName: 'Max',
+    lastName: 'Mustermann',
+    className: '10a',
+    phone: '0123 456789',
+  );
+}