phase3(foundation): http_util dio5 migration + null-safe models

- http_util.dart: dio4->5 (DioException, nullable params, drop unused statusCode,
  null-safe e.response!.data), countryCode ?? ''
- models/* (39 files): make all instance fields nullable (fromJson initializer-list
  pattern forces nullable); fix internal method usages (cart_info/cart_line_item/
  order/shipping_rate) with !/??
- models/ now passes dart analyze with 0 errors

Widget-level null-safety + remaining utils/store continue in next batch.

lib/models/product.dart

@@ -5,23 +5,23 @@ import 'Subproduct.dart';
 import 'product_attribute.dart';
 
 class Product {
-  int id;
-  int businessId;
-  int categoryId;
-  String name;
-  double price;
-  double regularPrice;
-  String description;
-  String detailDescription;
-  String imagePath;
-  String secondImagePath;
-  double monthSales;
-  int rate;
-  double leftNum;
-  List<ProductAttribute> productAttributes;
-  bool nonInventory;
-  String extraData;
-  List<Subproduct> subproducts;
+  int? id;
+  int? businessId;
+  int? categoryId;
+  String? name;
+  double? price;
+  double? regularPrice;
+  String? description;
+  String? detailDescription;
+  String? imagePath;
+  String? secondImagePath;
+  double? monthSales;
+  int? rate;
+  double? leftNum;
+  List<ProductAttribute>? productAttributes;
+  bool? nonInventory;
+  String? extraData;
+  List<Subproduct>? subproducts;
 
   Product(int id, int businessId, String name, double price, String description,
       String imagePath, List<ProductAttribute> productAttributes) {