CmdPal: Allow mouse X1 button to navigate back (#40517)

## Summary of the Pull Request
Enables navigation to the previous page when the mouse X1 (back) button
is pressed. This improves user experience for those using multi-button
mice, making navigation more intuitive.

## PR Checklist

- [x] **Closes:** #40499
- [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:** nope
- [x] **New binaries:** nada
- [x] **Documentation updated:** nothing to add

## Detailed Description of the Pull Request / Additional comments

## Validation Steps Performed

.github/actions/spell-check/expect.txt

@@ -1932,6 +1932,7 @@ Wubi
 WUX
 Wwanpp
 XAxis
+XButton
 xclip
 xcopy
 XDeployment

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

@@ -14,8 +14,10 @@ using Microsoft.CommandPalette.Extensions;
 using Microsoft.Extensions.DependencyInjection;
 using Microsoft.PowerToys.Telemetry;
 using Microsoft.UI.Dispatching;
+using Microsoft.UI.Input;
 using Microsoft.UI.Xaml.Controls;
 using Microsoft.UI.Xaml.Controls.Primitives;
+using Microsoft.UI.Xaml.Input;
 using Microsoft.UI.Xaml.Media.Animation;
 using DispatcherQueue = Microsoft.UI.Dispatching.DispatcherQueue;
 
@@ -79,6 +81,8 @@ public sealed partial class ShellPage : Microsoft.UI.Xaml.Controls.Page,
         WeakReferenceMessenger.Default.Register<ShowToastMessage>(this);
         WeakReferenceMessenger.Default.Register<NavigateToPageMessage>(this);
 
+        AddHandler(PointerPressedEvent, new PointerEventHandler(ShellPage_OnPointerPressed), true);
+
         RootFrame.Navigate(typeof(LoadingPage), ViewModel);
     }
 
@@ -442,4 +446,24 @@ public sealed partial class ShellPage : Microsoft.UI.Xaml.Controls.Page,
             WeakReferenceMessenger.Default.Send<PerformCommandMessage>(new(commandViewModel.Model));
         }
     }
+
+    private void ShellPage_OnPointerPressed(object sender, PointerRoutedEventArgs e)
+    {
+        try
+        {
+            var ptr = e.Pointer;
+            if (ptr.PointerDeviceType == PointerDeviceType.Mouse)
+            {
+                var ptrPt = e.GetCurrentPoint(this);
+                if (ptrPt.Properties.IsXButton1Pressed)
+                {
+                    WeakReferenceMessenger.Default.Send(new NavigateBackMessage());
+                }
+            }
+        }
+        catch (Exception ex)
+        {
+            Logger.LogError("Error handling mouse button press event", ex);
+        }
+    }
 }