Add queued FFmpeg conversion and finish splitting the window from the host.

Co-authored-by: Cursor <cursoragent@cursor.com>

src/Explorer.Contracts/IWorkbenchHost.cs

@@ -53,6 +53,8 @@ public interface ITransferHost
         => Task.CompletedTask;
     Task EnqueueVerifyArchiveAsync(string archivePath, CancellationToken cancellationToken = default)
         => Task.CompletedTask;
+    Task EnqueueConvertAsync(string sourcePath, string destinationPath, ConversionKind kind, CancellationToken cancellationToken = default)
+        => Task.CompletedTask;
 }
 
 public interface ISourceHost