feat(todo): Upcoming's tasks dragged onto another day (ADR 0033, D16)

A row of Upcoming, overdue ones too, held and dropped on another day
moves there, its time, duration and recurrence kept; Moved to … offers
Undo. The tint over a day a drag hovers is laid beside the rows, not
around them, which paint on the Material beneath.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

apps/todo/halkyon_todo_flutter/test/app/todo_app_test.dart

@@ -1374,6 +1374,40 @@ void main() {
         expect(find.byKey(quickAddTitleKey), findsNothing);
       });
 
+      testWidgets('a row of Upcoming held and dropped on another day moves '
+          'there, its time kept, which can be undone', (tester) async {
+        final today = Day.of(DateTime.now());
+        await start(tester, usedBefore: true);
+        await seed(
+          tester,
+          TaskContent(
+            title: 'Water plants',
+            projectId: inbox(),
+            date: TaskDate(today, time: 23 * 60 + 30),
+          ),
+        );
+        await goTab(tester, 'Upcoming');
+        final day = find.byTooltip(RegExp('^Add a task on .*Tomorrow'));
+
+        await dragFrom(
+          tester,
+          tester.getCenter(row('Water plants')),
+          tester.getCenter(day) - const Offset(200, 0),
+        );
+
+        expect(
+          (await stored(tester))['Water plants']!.date,
+          TaskDate(today.plus(1), time: 23 * 60 + 30),
+        );
+        expect(find.textContaining('Moved to'), findsOneWidget);
+        await tester.tap(find.text('Undo'));
+        await tester.pumpAndSettle();
+        expect(
+          (await stored(tester))['Water plants']!.date,
+          TaskDate(today, time: 23 * 60 + 30),
+        );
+      });
+
       testWidgets('the add button dropped on a day in Upcoming adds a task '
           'for that day', (tester) async {
         final tomorrow = Day.of(DateTime.now()).plus(1);