fix(admin): preview public images through the admin thumb route when their URL fails

When a public image's thumb_url or url fails to load, the fileupload
field fetches its preview once through the authenticated thumb route
(session key in the header) and shows it from an object URL, revoked
when the item is forgotten and on unmount. The preview is reported as
unavailable only when that fetch or the object-URL image fails too.
Protected fields behave as before.

docs/services/storage.md

@@ -115,6 +115,8 @@ The route serves originals and thumbnails and answers 404 for a missing file, an
 
 With `serve` on, `public_path_prefix` must be an absolute URL path such as `/storage/uploads`: a leading slash, no scheme or host, no query, no empty, `.` or `..` segment and no `{` or `}`. Start-up fails with an error naming the key otherwise, for example for the WinterCMS disk path `storage/app/uploads/public` or a CDN URL. With `serve` off the key only builds file URLs, so a CDN origin is fine there.
 
+Admin fileupload previews do not depend on this route: when a public image's URL does not load, the admin falls back to its authenticated thumb route (see [File uploads](../backend/forms.md#file-uploads)).
+
 The route looks up the bucket and the database on each request: `route:list` lists it without opening either, and it answers 404 until `serve` has published them. The route owns the whole prefix, so start-up fails when a plugin registers a route at or under it.
 
 Without the key, the application decides where files are served: mount `attach.StaticHandlerPublic` itself, or put a web server or CDN in front of the bucket directory. Either way, serve uploads from a separate origin when you can; the [Attachments](../database/attachments.md) page explains why. The built-in route is meant for development and for deployments that cannot split the origin yet.