Slug inputs in the site editor now filter as you type via slugify.js
(built on the transliteration npm package: unicode incl. asian scripts
folds to ASCII, spaces become hyphens, anything outside [a-z0-9-] is
dropped) instead of erroring on commit. A new page left with an empty
slug gets one derived from its title. Server-side, _check_reserved
additionally rejects reserved root file names (robots.txt, ads.txt,
sitemap.xml, openapi.json, favicon.ico, site.webmanifest) with a
human-readable reason, which the editor surfaces in its error line so
the row can be edited and committed again; those URLs are also never
looked up as content when serving.
