fix(ui): diagnostic error surfacing on attachment drop

Opening the dropped IStorageFile stream ran outside the try, so a
first-drop failure escaped the async-void handler as an unobserved fault
(generic "An error occurred"). Wrap the stream-open loop and surface the
exception type via DropStatus; include the exception type in the generic
add-file failure too, so the intermittent case is analyzable.

src/ClaudeDo.Ui/ViewModels/Islands/DetailsIslandViewModel.cs

@@ -1099,7 +1099,8 @@ public sealed partial class DetailsIslandViewModel : ViewModelBase, IDisposable
             }
             catch (Exception ex)
             {
-                failures.Add($"{fileName}: {ex.Message}");
+                // Keep the exception type in the message so an intermittent failure is analyzable.
+                failures.Add($"{fileName}: {ex.GetType().Name}: {ex.Message}");
             }
         }