CmdPal: Hide search box on content pages [experiment] (#41479)

## Summary of the Pull Request

CmdPal now displays the search box only on pages that derive from
ListPage. On ContentPage (forms, etc.), the search box is hidden.

- Moves keyboard shortcut handling from SearchBox to ShellPage so
shortcuts are always handled.
- Keeps the search box hidden/disabled to preserve layout metrics. 
- Refines focus management to prevent focus jumps during navigation. 
- For ContentPage page's content gains focus automatically (not just
form inputs, but now markdown content as well - so you can scroll
immediately, for example).
- Adds accessibility (a11y) tweaks: when navigating to content pages
without a visible search box, sets an explicit focus target so screen
readers announce a meaningful element. Screen reader will now announce
navigation to the page.
- Adds a title to the main list page - "Home".


https://github.com/user-attachments/assets/f60d0826-df1f-468e-8e41-0266cd27878b


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## PR Checklist

- [x] Closes: #38967
- [ ] **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

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## Detailed Description of the Pull Request / Additional comments

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## Validation Steps Performed

src/modules/cmdpal/Microsoft.CmdPal.UI/ExtViews/ListPage.xaml.cs

@@ -30,7 +30,7 @@ public sealed partial class ListPage : Page,
 {
     private InputSource _lastInputSource;
 
-    private ListViewModel? ViewModel
+    internal ListViewModel? ViewModel
     {
         get => (ListViewModel?)GetValue(ViewModelProperty);
         set => SetValue(ViewModelProperty, value);