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>

MainPage.xaml

@@ -113,7 +113,8 @@
 
             <StackPanel Grid.Column="1" Orientation="Horizontal" Spacing="10">
                 <Button x:Name="OpenSettingsButton" Content="&#xE713;" FontFamily="Segoe Fluent Icons, Segoe MDL2 Assets"
-                        Click="OpenSettings_Click" Style="{StaticResource IconButtonStyle}" ToolTipService.ToolTip="Settings" />
+                        Click="OpenSettings_Click" Style="{StaticResource IconButtonStyle}"
+                        ToolTipService.ToolTip="Settings" AutomationProperties.Name="Settings" />
                 <Button x:Name="CancelButton" Content="Cancel" Click="Cancel_Click" IsEnabled="False" />
                 <Button x:Name="RenderButton" Content="Start render" Click="Render_Click" Style="{StaticResource PrimaryButtonStyle}" />
             </StackPanel>