feat: pass item aspects to eBay and set shipping defaults on bulk import

- Task 8: Convert aspects Dictionary<string,string> to Dictionary<string,string[]> in CreateInventoryItemAsync
- Task 9: Set Postage to RoyalMailTracked48 and ShippingCost to 3.49m on BulkImportRow.ToListingDraft()

EbayListingTool/Models/BulkImportRow.cs

@@ -124,7 +124,9 @@ public class BulkImportRow : INotifyPropertyChanged
             PhotoPaths = PhotoPaths.Split(',', StringSplitOptions.RemoveEmptyEntries)
                                    .Select(x => x.Trim())
                                    .Where(x => !string.IsNullOrEmpty(x))
-                                   .ToList()
+                                   .ToList(),
+            Postage = PostageOption.RoyalMailTracked48,
+            ShippingCost = 3.49m
         };
     }
 

EbayListingTool/Services/EbayListingService.cs

@@ -317,7 +317,9 @@ public class EbayListingService
                 title       = draft.Title,
                 description = draft.Description,
                 imageUrls   = imageUrls.Count > 0 ? imageUrls : null,
-                aspects     = (object?)null
+                aspects     = draft.Aspects.Count > 0
+                    ? draft.Aspects.ToDictionary(kv => kv.Key, kv => new[] { kv.Value })
+                    : (object?)null
             }
         };