simplified code across the app and consolidated duplicated helpers

lib/view/pages/timetable/custom_events/custom_event_edit_dialog.dart

@@ -13,6 +13,7 @@ import '../../../../widget/async_action_button.dart';
 import '../../../../widget/demo_restricted.dart';
 import '../../../../widget/focus_behaviour.dart';
 import '../data/subject_color_palette.dart';
+import '../data/timetable_appointment_factory.dart';
 import '../subject_colors/subject_color_picker.dart';
 
 class CustomEventEditDialog extends StatefulWidget {
@@ -72,9 +73,10 @@ class _CustomEventEditDialogState extends State<CustomEventEditDialog> {
   void initState() {
     super.initState();
     if (_isEditing) {
-      final s = widget.existingEvent!.startDate;
-      final e = widget.existingEvent!.endDate;
-      _isAllDay = isAllDayConvention(s, e);
+      final existing = widget.existingEvent!;
+      final s = existing.startDate;
+      final e = existing.endDate;
+      _isAllDay = TimetableAppointmentFactory.isCustomEventAllDay(existing);
       if (_isAllDay) {
         _startTime = _defaultStart;
         _endTime = _defaultEnd;
@@ -119,21 +121,6 @@ class _CustomEventEditDialogState extends State<CustomEventEditDialog> {
     return (fromMin(start), fromMin(end));
   }
 
-  /// All-day convention shared with [TimetableAppointmentFactory]: a custom
-  /// event is treated as all-day when its start and end both land on midnight
-  /// of the same day. We piggyback on this so we don't need a backend schema
-  /// change.
-  static bool isAllDayConvention(DateTime start, DateTime end) =>
-      start.year == end.year &&
-      start.month == end.month &&
-      start.day == end.day &&
-      start.hour == 0 &&
-      start.minute == 0 &&
-      start.second == 0 &&
-      end.hour == 0 &&
-      end.minute == 0 &&
-      end.second == 0;
-
   Future<void> _save() async {
     if (guardDemoAction(context)) return;
     if (_name.text.trim().isEmpty) {
@@ -142,7 +129,7 @@ class _CustomEventEditDialogState extends State<CustomEventEditDialog> {
 
     // All-day convention: store start and end as midnight of the chosen day.
     // The factory recognises this on read.
-    final midnight = DateTime(_date.year, _date.month, _date.day);
+    final midnight = _date.dateOnly;
     final startDate = _isAllDay ? midnight : _date.withTime(_startTime);
     final endDate = _isAllDay ? midnight : _date.withTime(_endTime);