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.

Models/ShortSettings.cs

@@ -25,9 +25,10 @@ public enum ShortStyle
     Video,
 }
 
-/// <summary>How the video style crops the Amiga source before overlaying it.</summary>
+/// <summary>How the Amiga source is cropped before it is scaled into the output.</summary>
 public enum ShortSourceCropMode
 {
+    None,
     Auto,
     Center,
 }