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/located_address.dart

@@ -1,17 +1,17 @@
 import 'dart:convert';
 
 class LocatedAddress {
-  int storeId;
-  double latitude;
-  double longitude;
-  String streetNumber;
-  String streetName;
-  String city;
-  String locality;
-  String province;
-  String country;
-  String postalCode;
-  String formattedAddress;
+  int? storeId;
+  double? latitude;
+  double? longitude;
+  String? streetNumber;
+  String? streetName;
+  String? city;
+  String? locality;
+  String? province;
+  String? country;
+  String? postalCode;
+  String? formattedAddress;
 
   LocatedAddress.fromJson(Map<String, dynamic> json)
     : storeId = json['store_id'],