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/api_buy.js

@@ -0,0 +1,13 @@
+const { chromium } = require('playwright');
+(async () => {
+  const browser = await chromium.launch({ headless: true });
+  const page = await (await browser.newContext()).newPage();
+  await page.goto('http://127.0.0.1:8099/', { waitUntil: 'load' });
+  await page.waitForTimeout(3000);
+  const data = await page.evaluate(async () => {
+    const r = await fetch('https://api.minipos.us/v1/get-buy-service-info/pos/1?store_id=0');
+    return JSON.stringify(await r.json());
+  }).catch(e => 'ERR: ' + e.message);
+  console.log('buy-service API:', data.slice(0, 600));
+  await browser.close();
+})();