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);
     });
   }
 }

lib/widgets/desktop/create_online_store_1.dart

@@ -411,7 +411,7 @@ class CreateOnlineStore1State extends State<CreateOnlineStore1> {
       print('value: $value');
       if (value['status'] == 'OK') {
         domainResult = (value['domain_check_result'] as List);
-        setState(() {});
+        if (mounted) setState(() {});
       } else if (value['status'] == 'ERROR') {
         String errors = '';
         for (int i = 0; i < (value['errors'] as List).length; i++) {

lib/widgets/desktop/desktop_download_apps.dart

@@ -55,6 +55,7 @@ class DesktopDownloadAppsState extends State<DesktopDownloadApps> {
     }
     col.children.add(wrap);
     Widget view = SingleChildScrollView(
+      primary: true,
       child: Row(
         children: [
           Container(