Slug input filtering, title-derived slugs, reserved file names

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.

docs/design-principles.md

@@ -32,8 +32,11 @@ evolves.
   lives under `/_/` (the API at `/_/api/`, uploaded files at `/_/f/`, built
   assets at `/_/assets/`, and the admin shell at `/_/admin`). The only
   other reserved root path is `/favicon.ico`, served from the build.
-  Slugs may not begin with `_` or `.` anywhere in the path — such URLs
-  are never looked up as content.
+  Slugs are lowercase ASCII (`[a-z0-9-]`; input is transliterated and
+  filtered as you type, and a new page's empty slug is derived from its
+  title), may not begin with `_` or `.`, and may not occupy a reserved
+  root file name (`robots.txt`, `sitemap.xml`, `favicon.ico`, …) — such
+  URLs are never looked up as content.
 - **Single user, trusted author.** No auth concerns in the core design.
   Everything published is public; only editing tools will later sit behind
   access control (external SSO when that time comes). The author is trusted