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/shipping_rate.dart

@@ -4,9 +4,9 @@ import 'dart:convert';
 import 'package:flutter/foundation.dart';
 
 class ShippingRate {
-  String name;
-  double price;
-  List<Rate> rates;
+  String? name;
+  double? price;
+  List<Rate>? rates;
 
   ShippingRate.fromJson(Map<String, dynamic> json)
     : name = json['name'],
@@ -25,7 +25,7 @@ class ShippingRate {
           other is ShippingRate &&
           runtimeType == other.runtimeType &&
           name == other.name &&
-          price.toStringAsFixed(2) == other.price.toStringAsFixed(2) &&
+          price?.toStringAsFixed(2) == other.price?.toStringAsFixed(2) &&
           listEquals(rates, other.rates);
 
   @override
@@ -41,8 +41,8 @@ class ShippingRate {
 }
 
 class Rate {
-  String name;
-  double rate;
+  String? name;
+  double? rate;
 
   Rate.fromJson(Map<String, dynamic> json)
     : name = json['name'],
@@ -59,7 +59,7 @@ class Rate {
           other is Rate &&
           runtimeType == other.runtimeType &&
           name == other.name &&
-          rate.toStringAsFixed(2) == other.rate.toStringAsFixed(2);
+          rate?.toStringAsFixed(2) == other.rate?.toStringAsFixed(2);
 
   @override
   String toString() {