Commit the site's browser checks so reviewers and CI can rerun them #960

docs/standards/testing.md asks for a real-browser check of wizard and site layout changes, because tests/site/ runs the wizard in a DOM shim with no CSS, layout, focus or real events. Those checks have been run by hand with headless Chrome over the DevTools protocol, and the scripts were not committed, so a reviewer cannot rerun them and CI does not catch a regression such as a page laid out wider than a phone (#953).

The checks that found real defects:

• page width and content cut off by an overflow: hidden box, for every page in all six languages at 320, 375, 768 and 1280 px, after each page has applied its language (guide.html resets the language to English, so a sequential run must set it per page);
• every wizard control named by its visible caption (accessible name), in every step and scenario;
• focus after a re-render and after another tab's update, two real tabs included;
• contrast of focus rings and error text in both themes.

Acceptance criteria

• The layout check (page width and clipped content per page, language and width) is committed under tests/ or tools/, with the browser it needs pinned, and runs in CI or nightly.
• The accessible-name and focus checks are committed and runnable with one command; testing.md names that command instead of describing a manual check.
• A deliberately reintroduced grid-template-columns: auto 1fr on .step fails the layout check.