Template authoring

Add a template page without breaking tokens or the build.

1. Add an entry to site/scripts/nav.mjs with group: 'templates' and layout: 'page' or 'app'.
2. Create site/src/pages/<slug>.html — body fragment only.
3. Marketing pages use layouts/page.html + page.css. App previews use layouts/app.html + App Shell markup.
4. Run npm run build in site/. The page appears in catalog.json and the Templates hub automatically.

Do not copy docs.css into product or templates — it is documentation chrome only.