refactor(ui): remove unused Sort button from MyDay header

It was a no-op placeholder command; removed the button, command,
locale keys, and now-unused Icon.Sort geometry.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/ClaudeDo.Ui/ViewModels/Islands/TasksIslandViewModel.cs

@@ -706,9 +706,6 @@ public sealed partial class TasksIslandViewModel : ViewModelBase
     [RelayCommand]
     private void ToggleShowCompleted() => IsShowingCompleted = !IsShowingCompleted;
 
-    [RelayCommand]
-    private void Sort() { /* placeholder — UI-only */ }
-
     public event EventHandler? OpenListSettingsRequested;
 
     [RelayCommand]