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/probe4.js

@@ -0,0 +1,27 @@
+const { chromium } = require('playwright');
+const BASE = 'http://127.0.0.1:8099';
+(async () => {
+  const browser = await chromium.launch({ headless: true });
+  const page = await (await browser.newContext({ viewport: { width: 1280, height: 900 } })).newPage();
+  await page.goto(BASE + '/login', { waitUntil: 'load', timeout: 60000 });
+  await page.waitForTimeout(5000);
+
+  // JS 直接触发 placeholder click（绕过 viewport 限制）
+  await page.evaluate(() => {
+    const el = document.querySelector('flt-semantics-placeholder');
+    if (el) el.click();
+  });
+  await page.waitForTimeout(4000);
+
+  const textbox = await page.locator('[role="textbox"]').count();
+  const input = await page.locator('input').count();
+  console.log(`JS click 后: role=textbox=${textbox}, input=${input}`);
+
+  const labels = await page.evaluate(() =>
+    [...document.querySelectorAll('[role="textbox"],[role="button"]')]
+      .map(e => ({ role: e.getAttribute('role'), label: e.getAttribute('aria-label') }))
+      .filter(x => x.label && x.label.length < 50).slice(0, 30)
+  );
+  console.log('labels:', JSON.stringify(labels));
+  await browser.close();
+})().catch(e => { console.error('失败:', e.message); process.exit(1); });