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


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

<!-- 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

@@ -122,31 +122,11 @@ public sealed partial class SearchBar : UserControl,
             return;
         }
 
-        var ctrlPressed = InputKeyboardSource.GetKeyStateForCurrentThread(VirtualKey.Control).HasFlag(CoreVirtualKeyStates.Down);
-        var altPressed = InputKeyboardSource.GetKeyStateForCurrentThread(VirtualKey.Menu).HasFlag(CoreVirtualKeyStates.Down);
-        var shiftPressed = InputKeyboardSource.GetKeyStateForCurrentThread(VirtualKey.Shift).HasFlag(CoreVirtualKeyStates.Down);
-        var winPressed = InputKeyboardSource.GetKeyStateForCurrentThread(VirtualKey.LeftWindows).HasFlag(CoreVirtualKeyStates.Down) ||
-            InputKeyboardSource.GetKeyStateForCurrentThread(VirtualKey.RightWindows).HasFlag(CoreVirtualKeyStates.Down);
-        if (ctrlPressed && e.Key == VirtualKey.Enter)
-        {
-            // ctrl+enter
-            WeakReferenceMessenger.Default.Send<ActivateSecondaryCommandMessage>();
-            e.Handled = true;
-        }
-        else if (e.Key == VirtualKey.Enter)
-        {
-            WeakReferenceMessenger.Default.Send<ActivateSelectedListItemMessage>();
-            e.Handled = true;
-        }
-        else if (ctrlPressed && e.Key == VirtualKey.K)
-        {
-            // ctrl+k
-            WeakReferenceMessenger.Default.Send<OpenContextMenuMessage>(new OpenContextMenuMessage(null, null, null, ContextMenuFilterLocation.Bottom));
-            e.Handled = true;
-        }
-        else if (ctrlPressed && e.Key == VirtualKey.I)
+        var ctrlPressed = (InputKeyboardSource.GetKeyStateForCurrentThread(VirtualKey.Control) & CoreVirtualKeyStates.Down) == CoreVirtualKeyStates.Down;
+        if (ctrlPressed && e.Key == VirtualKey.I)
         {
             // Today you learned that Ctrl+I in a TextBox will insert a tab
+            // We don't want that, so we'll suppress it, this way it can be used for other purposes
             e.Handled = true;
         }
         else if (e.Key == VirtualKey.Escape)
@@ -280,22 +260,6 @@ public sealed partial class SearchBar : UserControl,
             _inSuggestion = false;
             _lastText = null;
         }
-
-        if (!e.Handled)
-        {
-            var ctrlPressed = InputKeyboardSource.GetKeyStateForCurrentThread(VirtualKey.Control).HasFlag(CoreVirtualKeyStates.Down);
-            var altPressed = InputKeyboardSource.GetKeyStateForCurrentThread(VirtualKey.Menu).HasFlag(CoreVirtualKeyStates.Down);
-            var shiftPressed = InputKeyboardSource.GetKeyStateForCurrentThread(VirtualKey.Shift).HasFlag(CoreVirtualKeyStates.Down);
-            var winPressed = InputKeyboardSource.GetKeyStateForCurrentThread(VirtualKey.LeftWindows).HasFlag(CoreVirtualKeyStates.Down) ||
-                InputKeyboardSource.GetKeyStateForCurrentThread(VirtualKey.RightWindows).HasFlag(CoreVirtualKeyStates.Down);
-
-            // The CommandBar is responsible for handling all the item keybindings,
-            // since the bound context item may need to then show another
-            // context menu
-            TryCommandKeybindingMessage msg = new(ctrlPressed, altPressed, shiftPressed, winPressed, e.Key);
-            WeakReferenceMessenger.Default.Send(msg);
-            e.Handled = msg.Handled;
-        }
     }
 
     private void FilterBox_PreviewKeyUp(object sender, KeyRoutedEventArgs e)