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/RenderSettings.cs

@@ -29,6 +29,11 @@ public sealed record RenderSettings
     public required double EndCardHoldSeconds { get; init; }
     public required int ThumbnailInterval { get; init; }
     public bool MoveSourcesToOriginals { get; init; } = true;
+    public ShortSourceCropMode SourceCropMode { get; init; } = ShortSourceCropMode.Auto;
+    public int CropLeft { get; init; }
+    public int CropRight { get; init; }
+    public int CropTop { get; init; }
+    public int CropBottom { get; init; }
     public int Width { get; init; } = 3840;
     public int Height { get; init; } = 2160;
     public int FramesPerSecond { get; init; } = 50;
@@ -37,3 +42,13 @@ public sealed record RenderSettings
 }
 
 public sealed record RenderProgress(double Percent, string Stage, string Message);
+
+/// <summary>Which moment of a long-form video a preview still represents.</summary>
+public enum VideoPreviewShot
+{
+    Opening,
+    Closing,
+}
+
+/// <summary>PNG stills of the cropped 4K source look and the end card.</summary>
+public sealed record VideoPreviewFrames(string OpeningPath, string ClosingPath);