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

<!-- Enter a brief description/summary of your PR here. What does it
fix/what does it change/how was it tested (even manually, if necessary)?
-->
## Summary of the Pull Request

<!-- Please review the items on the PR checklist before submitting-->
## PR Checklist

- [x] **Closes:** #40515 
- [ ] **Communication:** I've discussed this with core contributors
already. If the work hasn't been agreed, this work might be rejected
- [ ] **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
- [ ] [JSON for
signing](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ESRPSigning_core.json)
for new binaries
- [ ] [WXS for
installer](https://github.com/microsoft/PowerToys/blob/main/installer/PowerToysSetup/Product.wxs)
for new binaries and localization folder
- [ ] [YML for CI
pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ci/templates/build-powertoys-steps.yml)
for new test projects
- [ ] [YML for signed
pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/release.yml)
- [ ] **Documentation updated:** If checked, please file a pull request
on [our docs
repo](https://github.com/MicrosoftDocs/windows-uwp/tree/docs/hub/powertoys)
and link it here: #xxx

<!-- Provide a more detailed description of the PR, other things fixed,
or any additional comments/features here -->
## Detailed Description of the Pull Request / Additional comments

<!-- Describe how you validated the behavior. Add automated tests
wherever possible, but list manual validation steps taken as well -->
## 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)
         {