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/probe3.js

@@ -0,0 +1,40 @@
+// probe3：开启 Flutter web semantics（Enable accessibility），看能否得到 DOM 输入框
+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);
+
+  // 方法1：点击 flt-semantics-placeholder（"Enable accessibility"）
+  const ph = await page.locator('flt-semantics-placeholder').count();
+  console.log('placeholder 数:', ph);
+  if (ph > 0) {
+    await page.locator('flt-semantics-placeholder').click({ force: true }).catch(e => console.log('点击 placeholder 失败:', e.message.slice(0,60)));
+    await page.waitForTimeout(3000);
+  }
+
+  // 方法2：尝试 JS 开启（多种已知方式）
+  await page.evaluate(() => {
+    // 部分版本支持
+    if (window._flutter) { try { window._flutter.semanticsEnabled = true; } catch(e){} }
+  });
+
+  const textbox = await page.locator('[role="textbox"]').count();
+  const btn = await page.locator('[role="button"]').count();
+  const input = await page.locator('input').count();
+  console.log(`开启后: role=textbox=${textbox}, role=button=${btn}, input=${input}`);
+
+  // 列出 semantics 元素的 label（找用户名/密码/登录按钮）
+  const labels = await page.evaluate(() => {
+    return [...document.querySelectorAll('[role="textbox"], [role="button"], [aria-label]')]
+      .map(e => ({ role: e.getAttribute('role'), label: e.getAttribute('aria-label'), name: e.tagName }))
+      .filter(x => x.label && x.label.length < 40)
+      .slice(0, 25);
+  });
+  console.log('semantics labels:', JSON.stringify(labels));
+
+  await page.screenshot({ path: 'login_semantics.png' });
+  await browser.close();
+})().catch(e => { console.error('失败:', e.message); process.exit(1); });