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>

Package.appxmanifest

@@ -5,8 +5,7 @@
   xmlns:mp="http://schemas.microsoft.com/appx/2014/phone/manifest"
   xmlns:uap="http://schemas.microsoft.com/appx/manifest/uap/windows10"
   xmlns:rescap="http://schemas.microsoft.com/appx/manifest/foundation/windows10/restrictedcapabilities"
-  xmlns:systemai="http://schemas.microsoft.com/appx/manifest/systemai/windows10"
-  IgnorableNamespaces="uap rescap systemai">
+  IgnorableNamespaces="uap rescap">
 
   <Identity
     Name="F004AE73-5989-46F3-B913-E9A3A355713F"
@@ -48,6 +47,5 @@
 
   <Capabilities>
     <rescap:Capability Name="runFullTrust" />
-    <systemai:Capability Name="systemAIModels"/>
   </Capabilities>
 </Package>