fix(runtime): e2e 发现的 3 个 bug 修复

- download: desktop_download_apps SingleChildScrollView 加 primary:true (Scrollbar 缺 ScrollPosition)
- contact-stores: checkDomainAvailability async 后 setState 加 mounted 保护 (dispose 后 setState)
- buy-service: _loadData 加 data 结构校验 + mounted 保护 (API 400 无效参数时 data 为错误结构 -> RangeError)

lib/pages/buy_service.dart

@@ -72,12 +72,16 @@ class BuyServiceState extends State<BuyService> {
         'store_id': '${widget.sid}',
       }
     ).then((value) {
+      if (!mounted) return;
+      if (value == null || value is! Map || value['service_selections'] == null) {
+        Utils.showMessageDialog(context, 'Service info unavailable');
+        return;
+      }
       data = value;
       data!['domain'] = widget.domain;
-      print('data: $data');
       setState(() {});
     }).onError((error, stackTrace) {
-      Utils.showMessageDialog(context, error);
+      if (mounted) Utils.showMessageDialog(context, error);
     });
   }
 }