Add Shorts look preview and visual crop editor

Show opening/closing 9:16 stills from the same filtergraph, and let
users drag a keep-region on a source frame to fill the crop insets.

Models/ShortSettings.cs

@@ -32,6 +32,16 @@ public enum ShortSourceCropMode
     Center,
 }
 
+/// <summary>Which moment of the Short a preview still represents.</summary>
+public enum ShortPreviewShot
+{
+    Opening,
+    Closing,
+}
+
+/// <summary>PNG stills of the opening hook and closing title card.</summary>
+public sealed record ShortPreviewFrames(string OpeningPath, string ClosingPath);
+
 /// <summary>One YouTube Shorts render job's parameters (port of amigadb-short-fx-v4.sh).</summary>
 public sealed record ShortSettings
 {