debug: log crash_log.txt with full stack trace on any unhandled exception or Post failure

EbayListingTool/App.xaml.cs

@@ -1,4 +1,4 @@
-using System.Threading.Tasks;
+using System.Threading.Tasks;
 using System.Windows;
 using System.Windows.Threading;
 using Microsoft.Extensions.Configuration;

EbayListingTool/Views/NewListingView.xaml.cs

@@ -671,7 +671,22 @@ public partial class NewListingView : UserControl
             SetState(ListingState.Success);
             GetWindow()?.SetStatus($"Listed: {_draft.Title}");
         }
-        catch (Exception ex) { ShowError("Post Failed", ex.Message); }
+        catch (Exception ex)
+        {
+            // Log full stack trace to help diagnose crashes
+            try
+            {
+                var logPath = System.IO.Path.Combine(
+                    Environment.GetFolderPath(Environment.SpecialFolder.ApplicationData),
+                    "EbayListingTool", "crash_log.txt");
+                var msg = $"{DateTime.Now:HH:mm:ss} [Post_Click] {ex.GetType().Name}: {ex.Message}\n{ex.StackTrace}\n";
+                if (ex.InnerException != null)
+                    msg += $"  Inner: {ex.InnerException.GetType().Name}: {ex.InnerException.Message}\n";
+                System.IO.File.AppendAllText(logPath, msg + "\n");
+            }
+            catch { }
+            ShowError("Post Failed", ex.Message);
+        }
         finally { SetPostBusy(false); }
     }