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.dart

@@ -2,22 +2,22 @@
 import 'dart:convert';
 
 class User {
-  int id;
-  String username;
-  String nickname;
-  String mobile;
-  String email;
-  String avatarUrl;
-  int lastAddressId;
-  String passCode;
-  double credit;
-  double wallet;
-  int coupon;
-  int points;
-  int orderNum;
-  double pointsToCreditsConversionRate;
-  String contactNumber;
-  String flutterMiniStoreToken;
+  int? id;
+  String? username;
+  String? nickname;
+  String? mobile;
+  String? email;
+  String? avatarUrl;
+  int? lastAddressId;
+  String? passCode;
+  double? credit;
+  double? wallet;
+  int? coupon;
+  int? points;
+  int? orderNum;
+  double? pointsToCreditsConversionRate;
+  String? contactNumber;
+  String? flutterMiniStoreToken;
 
   User.fromJson(Map<String, dynamic> json)
       : id = json['id'],