Code review cleanup: dead code, unused capability, accessibility gap

- Remove unused `using Microsoft.Win32;` from MainPage.xaml.cs (leftover
  from the WPF OpenFileDialog era; WinUI uses Windows.Storage.Pickers)
- Trim App.xaml.cs down to the two usings it actually needs instead of the
  full WinUI3 template boilerplate list
- Change App.MainWindowInstance to an internal setter (only MainWindow
  itself should assign it)
- Add AutomationProperties.Name to the icon-only Settings button so screen
  readers announce it (it only had a mouse tooltip before)
- Delete Assets/AppIcon.ico: unreferenced by both the manifest and code,
  the real app icon comes from ApplicationIcon (branding/AmiReel.ico) and
  AppWindow.SetIcon at runtime
- Drop the systemAIModels capability from Package.appxmanifest; the app
  doesn't call any Windows AI API, so declaring it is an unnecessary
  privilege request

Verified: dotnet build produces 0 warnings/errors, app launches unchanged.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

App.xaml.cs

@@ -1,20 +1,6 @@
-using Windows.ApplicationModel;
-using Windows.ApplicationModel.Activation;
-using Windows.Foundation;
-using Windows.Foundation.Collections;
-using Microsoft.UI.Xaml;
-using Microsoft.UI.Xaml.Controls;
-using Microsoft.UI.Xaml.Controls.Primitives;
-using Microsoft.UI.Xaml.Data;
-using Microsoft.UI.Xaml.Input;
-using Microsoft.UI.Xaml.Media;
-using Microsoft.UI.Xaml.Navigation;
-using Microsoft.UI.Xaml.Shapes;
+using Microsoft.UI.Xaml;
 using WinRT.Interop;
 
-// To learn more about WinUI, the WinUI project structure,
-// and more about our project templates, see: http://aka.ms/winui-project-info.
-
 namespace AmiReel;
 
 /// <summary>
@@ -24,8 +10,8 @@ public partial class App : Application
 {
     private Window? _window;
     public static IntPtr MainWindowHandle { get; private set; }
-    public static MainWindow? MainWindowInstance { get; set; }
-    
+    public static MainWindow? MainWindowInstance { get; internal set; }
+
     /// <summary>
     /// Initializes the singleton application object.  This is the first line of authored code
     /// executed, and as such is the logical equivalent of main() or WinMain().