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/comment.dart

@@ -4,15 +4,15 @@ import 'dart:convert';
 import 'product_image.dart';
 
 class Comment {
-  int id;
-  String contact;
-  String content;
-  int rating;
-  List<ProductImage> images;
-  String createdAt;
-  int thumbUp;
-  int thumbDown;
-  String replyFromStore;
+  int? id;
+  String? contact;
+  String? content;
+  int? rating;
+  List<ProductImage>? images;
+  String? createdAt;
+  int? thumbUp;
+  int? thumbDown;
+  String? replyFromStore;
 
   Comment.fromJson(Map<String, dynamic> json) :
       id = json['id'],