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)

tools/e2e/capture_data.js

@@ -0,0 +1,12 @@
+const { chromium } = require('playwright');
+(async () => {
+  const browser = await chromium.launch({ headless: true });
+  const page = await (await browser.newContext({viewport:{width:1280,height:900}})).newPage();
+  const logs = [];
+  page.on('console', m => logs.push(m.text()));
+  await page.goto('http://127.0.0.1:8099/buy-service/1/pos', { waitUntil: 'load' }).catch(()=>{});
+  await page.waitForTimeout(6000);
+  const dataLog = logs.filter(l => l.startsWith('data: {') || l.startsWith('data:{'));
+  console.log('应用 data:', dataLog[0] ? dataLog[0].slice(0, 500) : '(未捕获)');
+  await browser.close();
+})();