♻️ Refactor Vitest E2E tests to use shared test-site

- Add custom `loadPage` command to fetch and inject HTML content
- Start static server for test-site with dynamic port allocation
- Use real FluffyCloud test-site instead of inline HTML
- Screenshots now match other SDKs (Storybook, Static-Site, Ember, Ruby)

The `loadPage` command works around Vitest browser mode's limitation
of not supporting `page.goto()` by fetching HTML and using
`frame.setContent()` to inject it into the test iframe.

Author: Robdel12