added guardian login with views for their assigned childs

lib/state/app/modules/children/child_selection_cubit.dart

@@ -0,0 +1,33 @@
+import 'package:hydrated_bloc/hydrated_bloc.dart';
+
+import '../../../../api/marianumconnect/queries/get_capabilities/guardian_child.dart';
+
+/// The child a guardian is currently looking at. Shared by every module that
+/// shows per-child data (timetable, absence report, later messages), so
+/// switching the child in one place switches it everywhere.
+class ChildSelectionCubit extends HydratedCubit<String?> {
+  ChildSelectionCubit() : super(null);
+
+  void select(String childId) => emit(childId);
+
+  Future<void> reset() async => emit(null);
+
+  @override
+  String? fromJson(Map<String, dynamic> json) => json['childId'] as String?;
+
+  @override
+  Map<String, dynamic>? toJson(String? state) => {'childId': state};
+}
+
+/// The selected child if it is still linked, otherwise the first one. Null
+/// when there are no children.
+GuardianChild? effectiveChild(
+  List<GuardianChild> children,
+  String? selectedId,
+) {
+  if (children.isEmpty) return null;
+  for (final child in children) {
+    if (child.id == selectedId) return child;
+  }
+  return children.first;
+}