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_date_time.dart

@@ -4,10 +4,10 @@ import 'dart:convert';
 import 'booking_time.dart';
 
 class BookingDateTime {
-  String viewDate;
-  int unixTime;
-  String sendTimeTip;
-  List<BookingTime> bookTimes;
+  String? viewDate;
+  int? unixTime;
+  String? sendTimeTip;
+  List<BookingTime>? bookTimes;
 
   BookingDateTime(this.viewDate, this.unixTime, this.sendTimeTip, this.bookTimes);