fix(todo): the calendar's arrows stay put at the left

The month's Flexible and the Spacer shared what was left of the row, so
the arrows sat somewhere in the middle and moved as the way back to
today showed. They come first now, where nothing beside them moves them.

Whats-New: The calendar's arrows in Upcoming stay in one place at the left.
Whats-New-De: Die Pfeile im Kalender unter Demnächst bleiben links an ihrem Platz.

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

@@ -1049,6 +1049,36 @@ void main() {
         await done(tester);
       }
 
+      testWidgets('the arrows stay at the left, whatever the way back to '
+          'today or what is overdue beside them', (tester) async {
+        await start(tester, usedBefore: true);
+        await threeDay(tester);
+        final later = find.byTooltip('Later days');
+        final atToday = tester.getTopLeft(later);
+        expect(
+          atToday.dx,
+          lessThan(tester.getTopLeft(find.byType(HourGrid)).dx + 80),
+        );
+
+        await tester.tap(later);
+        await tester.pumpAndSettle();
+        expect(find.byTooltip('Back to today'), findsOneWidget);
+        expect(tester.getTopLeft(later), atToday);
+
+        await tester.tap(find.byTooltip('Back to today'));
+        await tester.pumpAndSettle();
+        await seed(
+          tester,
+          TaskContent(
+            title: 'Late',
+            projectId: inbox(),
+            date: TaskDate(Day.of(DateTime.now()).plus(-3)),
+          ),
+        );
+        expect(find.byKey(HourGrid.overdueKey), findsOneWidget);
+        expect(tester.getTopLeft(later), atToday);
+      });
+
       testWidgets('3-day shows today and the next two days, a timed task as '
           'a block and one without a time all day; kept in the settings, '
           'sealed', (tester) async {