improve start of week calculation

lib/util/date_extension.dart

@@ -24,6 +24,7 @@ extension DateExtension on DateTime {
   }
 
   DateTime atStartOfWeek() {
-    return subtract(Duration(days: weekday - 1));
+    final date = subtract(Duration(days: weekday - 1));
+    return DateTime(date.year, date.month, date.day);
   }
 }

test/start_of_week_test.dart

+import 'package:better_untis/util/date_extension.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  final startOfMonday = DateTime(2023, 3, 20, 0, 0);
+
+  test('Start of week at start of monday', () {
+    final startOfWeek = startOfMonday.atStartOfWeek();
+
+    expect(startOfWeek, startOfMonday);
+  });
+
+  test('Start of week at end of tuesday', () {
+    final endOfTuesday = DateTime(2023, 3, 21, 23, 59);
+    final startOfWeek = endOfTuesday.atStartOfWeek();
+
+    expect(startOfWeek, startOfMonday);
+  });
+
+  test('Start of week at end of sunday', () {
+    final endOfSaturday = DateTime(2023, 3, 26, 23, 59);
+    final startOfWeek = endOfSaturday.atStartOfWeek();
+
+    expect(startOfWeek, startOfMonday);
+  });
+
+  test('Start of week at end of next monday', () {
+    final startOfNextMonday = DateTime(2023, 3, 27, 0, 0);
+    final startOfWeek = startOfNextMonday.atStartOfWeek();
+
+    expect(startOfWeek, startOfNextMonday);
+  });
+}