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/node_modules/playwright/lib/agents/playwright-test-plan.prompt.md

@@ -0,0 +1,9 @@
+---
+agent: playwright-test-planner
+description: Create test plan
+---
+
+Create test plan for "add to cart" functionality of my app.
+
+- Seed file: `${seedFile}`
+- Test plan: `specs/coverage.plan.md`