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/user_position.dart

@@ -2,8 +2,8 @@
 import 'dart:convert';
 
 class UserPosition {
-  double lat;
-  double lng;
+  double? lat;
+  double? lng;
 
   UserPosition.fromJson(Map<String, dynamic> json) :
       lat = double.parse(json['lat'].toString()),