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/Views/Islands/DetailsIslandView.axaml.cs

@@ -81,14 +81,28 @@ public partial class DetailsIslandView : UserControl
         if (items is null) return;
 
         var files = new List<(string FileName, System.IO.Stream Content)>();
-        foreach (var item in items)
+        try
         {
-            if (item is IStorageFile sf)
+            foreach (var item in items)
             {
-                var stream = await sf.OpenReadAsync();
-                files.Add((sf.Name, stream));
+                if (item is IStorageFile sf)
+                {
+                    var stream = await sf.OpenReadAsync();
+                    files.Add((sf.Name, stream));
+                }
             }
         }
+        catch (Exception ex)
+        {
+            // Opening the dropped stream can fail (shell IO, file lock) before AddFilesAsync
+            // ever runs; surface it with the exception type so the next occurrence is analyzable
+            // instead of vanishing as an unobserved async-void fault.
+            _vm.DropStatus = string.Format(
+                ClaudeDo.Ui.Localization.Loc.T("details.attachments.dropReadError"),
+                $"{ex.GetType().Name}: {ex.Message}");
+            foreach (var (_, s) in files) await s.DisposeAsync();
+            return;
+        }
 
         if (files.Count == 0) return;