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/booking_time.dart

@@ -2,9 +2,9 @@
 import 'dart:convert';
 
 class BookingTime {
-  String viewTime;
-  int unixTime;
-  String sendTimeTip;
+  String? viewTime;
+  int? unixTime;
+  String? sendTimeTip;
 
   BookingTime(this.viewTime, this.unixTime, this.sendTimeTip);