Transfers: say so when a drop or a download is refused because the screen is busy

tests/DodoSSH.Client.App.Tests/TransferQueueingTests.cs

@@ -79,6 +79,22 @@ public sealed class TransferQueueingTests : IDisposable
         transfers.Status.ShouldContain("Connect to a host first");
     }
 
+    /// <remarks>
+    /// A drop landing while another command on this screen was running used to vanish: nothing queued and
+    /// nothing said. It still queues nothing — the command in flight owns the pane — but it says so.
+    /// </remarks>
+    [Fact]
+    public void DroppingFilesWhileBusy_QueuesNothingAndSaysWhy()
+    {
+        Connected();
+        transfers.IsBusy = true;
+
+        transfers.QueueUploads([File("one.txt")]);
+
+        Queued().ShouldBeEmpty();
+        transfers.Status.ShouldContain("Nothing was queued");
+    }
+
     [Fact]
     public void DroppingSeveralFiles_QueuesEachOfThem()
     {