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/waiting_line.dart

@@ -4,8 +4,8 @@ import 'dart:convert';
 import 'waiting_line_item.dart';
 
 class WaitingLine {
-  List<WaitingLineItem> waitingLines;
-  String storeSn;
+  List<WaitingLineItem>? waitingLines;
+  String? storeSn;
 
   WaitingLine.fromJson(Map<String, dynamic> json)
     : waitingLines = (json['waiting_lines'] as List).map((i) => WaitingLineItem.fromJson(i)).toList(),