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/simple_product.dart

@@ -1,12 +1,12 @@
 import 'dart:convert';
 
 class SimpleProduct {
-  int id;
-  String name;
-  double price;
-  double regularPrice;
-  String description;
-  String imagePath;
+  int? id;
+  String? name;
+  double? price;
+  double? regularPrice;
+  String? description;
+  String? imagePath;
 
   SimpleProduct.fromJson(Map<String, dynamic> json)
     : id = json['id'],