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/order.dart

@@ -11,33 +11,33 @@ import 'fulfillment.dart';
 import 'user_position.dart';
 
 class Order {
-  int id;
-  int userId;
-  int businessId;
-  Business businessInfo;
-  CartInfo cartInfo;
-  int status;
-  double originPrice;
-  double discountPrice;
-  double totalPrice;
-  String consignee;
-  String phone;
-  String address;
-  Address shippingAddress;
-  int payMethod;
-  String remark;
-  String orderNum;
-  int bookedAt;
-  int createdAt;
-  String shippingMethod;
+  int? id;
+  int? userId;
+  int? businessId;
+  Business? businessInfo;
+  CartInfo? cartInfo;
+  int? status;
+  double? originPrice;
+  double? discountPrice;
+  double? totalPrice;
+  String? consignee;
+  String? phone;
+  String? address;
+  Address? shippingAddress;
+  int? payMethod;
+  String? remark;
+  String? orderNum;
+  int? bookedAt;
+  int? createdAt;
+  String? shippingMethod;
   String paymentStatus; // UNPAID, PAID, PARTIALPAID
-  double amountPaid;
-  List<Fulfillment> fulfillments;
-  String extraData;
-  bool hasComment;
-  double fee;
-  UserPosition shipperPosition;
-  DistanceInfo deliveryDistance;
+  double? amountPaid;
+  List<Fulfillment>? fulfillments;
+  String? extraData;
+  bool? hasComment;
+  double? fee;
+  UserPosition? shipperPosition;
+  DistanceInfo? deliveryDistance;
 
   Order.fromJson(Map<String, dynamic> json)
       : id = json['id'],
@@ -100,7 +100,7 @@ class Order {
 
   double getSubtotal() {
     double subtotal = 0.0;
-    for (CartLineItem lineItem in cartInfo.productList) {
+    for (CartLineItem lineItem in cartInfo?.productList ?? <CartLineItem>[]) {
       subtotal += lineItem.getTotalPrice();
     }
     return subtotal;