fix(test): update future task checkbox test for anytime completion

Phase 11 removed checkbox-disable restrictions so users can complete
tasks on any day. Updated test to expect enabled checkboxes instead
of disabled ones.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

test/features/tasks/presentation/task_list_screen_test.dart

@@ -246,8 +246,8 @@ void main() {
     });
   });
 
-  group('TaskListScreen future task restriction', () {
-    testWidgets('checkboxes are disabled for future tasks', (tester) async {
+  group('TaskListScreen future task completion', () {
+    testWidgets('checkboxes are enabled for future tasks', (tester) async {
       final futureDate = today.add(const Duration(days: 3));
       await tester.pumpWidget(_buildApp(CalendarDayState(
         selectedDate: futureDate,
@@ -268,9 +268,9 @@ void main() {
       // Task should be visible
       expect(find.text('Fenster putzen'), findsOneWidget);
 
-      // Checkbox should exist but be disabled (onChanged is null)
+      // Checkbox should be enabled (Phase 11: anytime completion)
       final checkbox = tester.widget<Checkbox>(find.byType(Checkbox));
-      expect(checkbox.onChanged, isNull);
+      expect(checkbox.onChanged, isNotNull);
     });
   });