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/cart_line_item.dart

@@ -6,15 +6,15 @@ import 'package:uuid/uuid.dart';
 import 'product.dart';
 
 class CartLineItem {
-  int id;
-  double unitPrice;
-  double totalPrice;
-  Product product;
-  String name;
-  String description;
-  double quantity;
-  String uuid;
-  String parentUuid;
+  int? id;
+  double? unitPrice;
+  double? totalPrice;
+  Product? product;
+  String? name;
+  String? description;
+  double? quantity;
+  String? uuid;
+  String? parentUuid;
 
   CartLineItem() {
     uuid = Uuid().v4();
@@ -48,7 +48,7 @@ class CartLineItem {
   }
 
   double getTotalPrice() {
-    totalPrice = unitPrice * quantity;
-    return totalPrice;
+    totalPrice = (unitPrice ?? 0.0) * (quantity ?? 0);
+    return totalPrice ?? 0.0;
   }
 }