CmdPal: Fix search box text selection in ShellPage.GoHome (#42937)

## Summary of the Pull Request

This PR fixes an issue where `ShellPage.GoHome` wouldn’t select the
search box text when the current page was already the home page.

In that case, the navigation stack was empty, and no code was executed
because focusing the text had been delegated to the `GoBack` operation.

## Change log one-liner

Ensured search text is selected when Go home when activated and
Highlight search on activate are both enabled.

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## PR Checklist

- [x] Closes: #42443
- [x] **Communication:** I've discussed this with core contributors
already. If the work hasn't been agreed, this work might be rejected
- [x] **Tests:** Added/updated and all pass
- [x] **Localization:** All end-user-facing strings can be localized
- [x] **Dev docs:** Added/updated
- [x] **New binaries:** Added on the required places
- [x] **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

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or any additional comments/features here -->
## Detailed Description of the Pull Request / Additional comments

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wherever possible, but list manual validation steps taken as well -->
## Validation Steps Performed

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

@@ -451,7 +451,15 @@ public sealed partial class ShellPage : Microsoft.UI.Xaml.Controls.Page,
     {
         while (RootFrame.CanGoBack)
         {
-            GoBack(withAnimation, focusSearch);
+            // don't focus on each step, just at the end
+            GoBack(withAnimation, focusSearch: false);
+        }
+
+        // focus search box, even if we were already home
+        if (focusSearch)
+        {
+            SearchBox.Focus(Microsoft.UI.Xaml.FocusState.Programmatic);
+            SearchBox.SelectSearch();
         }
     }