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/business.dart

@@ -8,7 +8,7 @@ import 'product.dart';
 import 'waiting_line.dart';
 
 class QuickInput {
-  String value;
+  String? value;
   QuickInput.fromJson(Map<String, dynamic> json)
     : value = json['value'];
 
@@ -23,48 +23,48 @@ class QuickInput {
 }
 
 class Business {
-  int id;
-  String name;
-  String phone;
-  Address address;
-  String fullAddress;
-  String picUrl;
-  int status;
-  List<BusinessTime> openingTime;
-  double shippingFee;
-  double packageFee;
-  double minPrice;
-  int shippingTime;
-  int monthSales;
-  String bulletin;
-  int category;
-  bool isLike;
-  String description;
-  String policy;
-  String bannerImageUrl;
-  int todayOpen;
-  int todayClose;
-  bool showMonthlySold;
-  List<ImageUrl> slideImages;
-  List<Product> promoProducts;
-  String paypalEmail;
-  bool deliveryCanadaPost;
-  bool deliveryStoreDelivery;
-  bool deliveryPickup;
-  String currency;
-  List<WaitingLine> waitingLine;
-  bool enableWaitingLine;
-  double pointsToCreditsConversionRate;
-  String chaseXLogin;
-  String chaseTransactionKey;
-  String chaseResponseKey;
-  DistanceInfo distanceInfo;
-  int commentsCount;
-  bool forceClose;
-  bool isPublic;
-  double referrerCoupon;
-  bool instanceDelivery;
-  List<QuickInput> quickInputs;
+  int? id;
+  String? name;
+  String? phone;
+  Address? address;
+  String? fullAddress;
+  String? picUrl;
+  int? status;
+  List<BusinessTime>? openingTime;
+  double? shippingFee;
+  double? packageFee;
+  double? minPrice;
+  int? shippingTime;
+  int? monthSales;
+  String? bulletin;
+  int? category;
+  bool? isLike;
+  String? description;
+  String? policy;
+  String? bannerImageUrl;
+  int? todayOpen;
+  int? todayClose;
+  bool? showMonthlySold;
+  List<ImageUrl>? slideImages;
+  List<Product>? promoProducts;
+  String? paypalEmail;
+  bool? deliveryCanadaPost;
+  bool? deliveryStoreDelivery;
+  bool? deliveryPickup;
+  String? currency;
+  List<WaitingLine>? waitingLine;
+  bool? enableWaitingLine;
+  double? pointsToCreditsConversionRate;
+  String? chaseXLogin;
+  String? chaseTransactionKey;
+  String? chaseResponseKey;
+  DistanceInfo? distanceInfo;
+  int? commentsCount;
+  bool? forceClose;
+  bool? isPublic;
+  double? referrerCoupon;
+  bool? instanceDelivery;
+  List<QuickInput>? quickInputs;
 
   Business.fromJson(Map<String, dynamic> json)
     : id = json['id'],