Set up CI for website e2e tests & fix stale cloud href assertion in download.spec.ts

## Summary

The website Playwright e2e tests under `apps/website/e2e/` are not currently running in CI. This was surfaced during review of PR #11273.

Additionally, the smoke test in `apps/website/e2e/download.spec.ts` (line 14) asserts a stale URL:

```ts
await expect(link).toHaveAttribute('href', 'https://app.comfy.org')
```

The `externalLinks.app` entry in `apps/website/src/config/routes.ts` was renamed to `externalLinks.cloud` and its value was updated to `https://cloud.comfy.org` in PR #11273. The test assertion needs to be updated to match:

```ts
await expect(link).toHaveAttribute('href', 'https://cloud.comfy.org')
```

## Action Items

- [ ] Configure a CI job to run `apps/website/e2e/` Playwright tests on pull requests
- [ ] Update the stale `href` assertion in `apps/website/e2e/download.spec.ts` line 14 from `https://app.comfy.org` to `https://cloud.comfy.org`

## References

- PR: https://github.com/Comfy-Org/ComfyUI_frontend/pull/11273
- Review comment: https://github.com/Comfy-Org/ComfyUI_frontend/pull/11273#discussion_r3090270798

Requested by @DrJKL

┆Issue is synchronized with this [Notion page](https://www.notion.so/Issue-11300-Set-up-CI-for-website-e2e-tests-fix-stale-cloud-href-assertion-in-download-spec-ts-3446d73d365081a7b936dfb5a4d1dc2e) by [Unito](https://www.unito.io)