[CmdPal] Tray icon settings (#38672)

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## Summary of the Pull Request

Added a settings to enable/disable the system tray icon (enabled by default).
Adopter the term "system tray icon" for consistency with Windows 11 settings.

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## PR Checklist

- [x] **Closes:** #38407

src/modules/cmdpal/Microsoft.CmdPal.UI.ViewModels/SettingsViewModel.cs

@@ -87,6 +87,16 @@ public partial class SettingsViewModel : INotifyPropertyChanged
         }
     }
 
+    public bool ShowSystemTrayIcon
+    {
+        get => _settings.ShowSystemTrayIcon;
+        set
+        {
+            _settings.ShowSystemTrayIcon = value;
+            Save();
+        }
+    }
+
     public ObservableCollection<ProviderSettingsViewModel> CommandProviders { get; } = [];
 
     public SettingsViewModel(SettingsModel settings, IServiceProvider serviceProvider, TaskScheduler scheduler)