The customCSS field on status pages was persisted and validated
end-to-end but never rendered on the public page, so operator-supplied
styles had no effect.
Inject the string as a <style> tag on the public route (which the
admin preview iframe also renders), reusing the existing
cssReferencesExternalResource check so external-resource CSS
(@import, url(http…), //…) is dropped defensively on the client.
The admin preview iframe already points at the public URL, so this
one insertion covers both the live page and the in-app preview.