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_detail.dart

@@ -8,30 +8,30 @@ import 'product_attribute.dart';
 import 'product_image.dart';
 
 class ProductDetail {
-  int id;
-  String image;
-  List<ProductImage> images;
-  String sku;
-  String name;
-  double price;
-  double regularPrice;
-  String description;
-  String description2;
-  String detailDescription;
-  double monthSales;
-  int rate;
-  double leftNum;
-  List<ProductAttribute> productAttributes;
-  bool nonInventory;
-  String extraData;
-  String purchaseNote;
-  double weight;
-  double dimentionsLength;
-  double dimentionsWidth;
-  double dimentionsHeight;
-  bool reviewsAllowed;
-  List<Category> categories;
-  List<Subproduct> subproducts;
+  int? id;
+  String? image;
+  List<ProductImage>? images;
+  String? sku;
+  String? name;
+  double? price;
+  double? regularPrice;
+  String? description;
+  String? description2;
+  String? detailDescription;
+  double? monthSales;
+  int? rate;
+  double? leftNum;
+  List<ProductAttribute>? productAttributes;
+  bool? nonInventory;
+  String? extraData;
+  String? purchaseNote;
+  double? weight;
+  double? dimentionsLength;
+  double? dimentionsWidth;
+  double? dimentionsHeight;
+  bool? reviewsAllowed;
+  List<Category>? categories;
+  List<Subproduct>? subproducts;
 
   ProductDetail.fromJson(Map<String, dynamic> json)
     : id = json['id'],