feat(receipt-import): enhance package handling with new fields and UI adjustments

flutter/lib/features/import/data/receipt_import_session.dart

@@ -18,6 +18,9 @@ class ItemEdit {
   final CategorySelectionSource? categorySource;
   final double? quantity;
   final String? unit;
+  final double? packQuantity;
+  final String? packUnit;
+  final double? packageCount;
   final ImportDestination destination;
 
   const ItemEdit({
@@ -28,6 +31,9 @@ class ItemEdit {
     this.categorySource,
     this.quantity,
     this.unit,
+    this.packQuantity,
+    this.packUnit,
+    this.packageCount,
     this.destination = ImportDestination.inventory,
   });
 }