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/blog.dart

@@ -2,8 +2,8 @@
 import 'dart:convert';
 
 class Section {
-  int id;
-  String name;
+  int? id;
+  String? name;
 
   Section.fromJson(Map<String, dynamic> json) :
       id = json['id'],
@@ -21,14 +21,14 @@ class Section {
 }
 
 class Blog {
-  int id;
-  String title;
-  String body;
-  String createdAt;
-  String thumbUrl;
-  String imageUrl;
-  String videoUrl;
-  Section section;
+  int? id;
+  String? title;
+  String? body;
+  String? createdAt;
+  String? thumbUrl;
+  String? imageUrl;
+  String? videoUrl;
+  Section? section;
 
   Blog.fromJson(Map<String, dynamic> json) :
       id = json['id'],