simplify: dedupe boilerplate and remove dead code across all layers

lib/view/pages/timetable/widgets/calendar/outside_chips.dart

@@ -116,12 +116,9 @@ class _OutsideDayColumn extends StatelessWidget {
 
   static String _subtitleFor(Appointment a) {
     if (isAllDayLike(a)) return 'Ganztägig';
-    return '${_hm(a.startTime)}–${_hm(a.endTime)}';
+    return '${a.startTime.formatHm()}–${a.endTime.formatHm()}';
   }
 
-  static String _hm(DateTime t) =>
-      '${t.hour.toString().padLeft(2, '0')}:${t.minute.toString().padLeft(2, '0')}';
-
   @override
   Widget build(BuildContext context) {
     if (appointments.isEmpty) return const SizedBox.shrink();
@@ -182,9 +179,7 @@ class _OutsideChip extends StatelessWidget {
   Widget build(BuildContext context) {
     final theme = Theme.of(context);
     final allDay = isAllDayLike(appointment);
-    final timeLabel = allDay
-        ? null
-        : '${appointment.startTime.hour.toString().padLeft(2, '0')}:${appointment.startTime.minute.toString().padLeft(2, '0')}';
+    final timeLabel = allDay ? null : appointment.startTime.formatHm();
 
     // Past chips fade further, future/ongoing ones get a more saturated tint
     // so the strip no longer reads as one uniform grey block.

lib/view/pages/timetable/widgets/calendar/week_grid.dart

@@ -289,14 +289,11 @@ class _DayColumn extends StatelessWidget {
   }
 
   static String _overflowSubtitle(Appointment apt) {
-    final time = '${_formatHm(apt.startTime)}–${_formatHm(apt.endTime)}';
+    final time = '${apt.startTime.formatHm()}–${apt.endTime.formatHm()}';
     final loc = apt.location?.replaceAll('\n', ' · ');
     return loc != null && loc.isNotEmpty ? '$time · $loc' : time;
   }
 
-  static String _formatHm(DateTime t) =>
-      '${t.hour.toString().padLeft(2, '0')}:${t.minute.toString().padLeft(2, '0')}';
-
   @override
   Widget build(BuildContext context) {
     final theme = Theme.of(context);