simplified code across the app and consolidated duplicated helpers

lib/view/pages/timetable/widgets/special_regions_builder.dart

@@ -44,11 +44,7 @@ class SpecialRegionsBuilder {
     for (var day = rangeStart; !day.isAfter(rangeEnd); day = day.addDays(1)) {
       if (holidayDays.contains(_dayKey(day))) continue;
       for (final p in breakPeriods) {
-        final start = day.copyWith(
-          hour: p.start.hour,
-          minute: p.start.minute,
-        );
-        breakRegions.add(_breakRegion(start, p.duration));
+        breakRegions.add(_breakRegion(day.withTime(p.start), p.duration));
       }
     }
 
@@ -64,16 +60,8 @@ class SpecialRegionsBuilder {
     // on top of each other.
     final byDay = <String, _HolidayDay>{};
     for (final holiday in holidays.result) {
-      final startDay = DateTime(
-        holiday.startDate.year,
-        holiday.startDate.month,
-        holiday.startDate.day,
-      );
-      final endDay = DateTime(
-        holiday.endDate.year,
-        holiday.endDate.month,
-        holiday.endDate.day,
-      );
+      final startDay = holiday.startDate.dateOnly;
+      final endDay = holiday.endDate.dateOnly;
       // Webuntis treats endDate inclusively (last day of the break) — the
       // `+ 1` covers single-day public holidays (where startDate == endDate)
       // and the final day of a multi-day vacation, both of which would