CmdPal: Handle Alt+Left Arrow even when search bar contains text (#40516)

Previously, the `KeyDown` handler for `Alt+Left Arrow` was not invoked
if the search bar contained text. By moving the logic to the
`PreviewKeyDown` handler, we ensure the shortcut is handled regardless
of the search bar’s content.

The handler was added to the `ShellPage`'s `PreviewKeyDown` handler,
rather than the one on `SearchBar`. This will allow the shortcut to be
available throughout the entire window and will ensure it works from
within the page content, e.g. from forms

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## Summary of the Pull Request

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## PR Checklist

- [x] **Closes:** #40515 
- [ ] **Communication:** I've discussed this with core contributors
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- [ ] **Tests:** Added/updated and all pass
- [ ] **Localization:** All end-user-facing strings can be localized
- [ ] **Dev docs:** Added/updated
- [ ] **New binaries:** Added on the required places
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## Validation Steps Performed

src/modules/cmdpal/Microsoft.CmdPal.UI/Controls/SearchBar.xaml.cs

@@ -162,10 +162,6 @@ public sealed partial class SearchBar : UserControl,
                 CurrentPageViewModel.Filter = FilterBox.Text;
             }
         }
-        else if (e.Key == VirtualKey.Left && altPressed)
-        {
-            WeakReferenceMessenger.Default.Send<NavigateBackMessage>(new());
-        }
 
         if (!e.Handled)
         {

src/modules/cmdpal/Microsoft.CmdPal.UI/Pages/ShellPage.xaml.cs

@@ -20,6 +20,7 @@ using Microsoft.UI.Xaml.Controls.Primitives;
 using Microsoft.UI.Xaml.Input;
 using Microsoft.UI.Xaml.Media.Animation;
 using DispatcherQueue = Microsoft.UI.Dispatching.DispatcherQueue;
+using VirtualKey = Windows.System.VirtualKey;
 
 namespace Microsoft.CmdPal.UI.Pages;
 
@@ -81,6 +82,7 @@ public sealed partial class ShellPage : Microsoft.UI.Xaml.Controls.Page,
         WeakReferenceMessenger.Default.Register<ShowToastMessage>(this);
         WeakReferenceMessenger.Default.Register<NavigateToPageMessage>(this);
 
+        AddHandler(PreviewKeyDownEvent, new KeyEventHandler(ShellPage_OnPreviewKeyDown), true);
         AddHandler(PointerPressedEvent, new PointerEventHandler(ShellPage_OnPointerPressed), true);
 
         RootFrame.Navigate(typeof(LoadingPage), ViewModel);
@@ -447,6 +449,14 @@ public sealed partial class ShellPage : Microsoft.UI.Xaml.Controls.Page,
         }
     }
 
+    private void ShellPage_OnPreviewKeyDown(object sender, KeyRoutedEventArgs e)
+    {
+        if (e.Key == VirtualKey.Left && e.KeyStatus.IsMenuKeyDown)
+        {
+            WeakReferenceMessenger.Default.Send<NavigateBackMessage>(new());
+        }
+    }
+
     private void ShellPage_OnPointerPressed(object sender, PointerRoutedEventArgs e)
     {
         try