Add video preview, source crop, find-videos, and render toasts

Bring the Video tab closer to Shorts with 4K preview stills and crop
modes (none, auto, center, plus the visual editor). Settings store a
default source folder so Find videos can collect output.avi sequences.
Cap the FFmpeg log with auto-scroll, hide repeat spam, and show a
Windows notification when a video or short finishes or fails.

App.xaml.cs

@@ -1,4 +1,5 @@
-using Microsoft.UI.Xaml;
+using AmiReel.Services;
+using Microsoft.UI.Xaml;
 using WinRT.Interop;
 
 namespace AmiReel;
@@ -29,6 +30,7 @@ public partial class App : Application
     {
         _window = new MainWindow();
         MainWindowHandle = WindowNative.GetWindowHandle(_window);
+        RenderNotifier.Register();
         _window.Activate();
     }
 }