Set the site address in the administration, checked against the installation itself

- System > This installation > Address has a field and a link to the other
  spelling (with or without www), like the WordPress address. It writes
  SITE_ORIGIN to .env through Environment::write and takes effect on the
  next request. With SITE_ORIGIN from the server environment there is no
  field, and a POST is refused.
- Before saving, OriginCheck requests /.well-known/kantsites-origin under the
  new address; it has to return an HMAC of APP_SECRET that only this
  installation knows. Switching to www without DNS or certificate would
  otherwise send every page, the administration included, nowhere. "Save
  anyway" appears only after a refusal. The probe also answers during
  maintenance.
- The message says when the administration changes host and needs a new
  sign-in, since the session cookie belongs to one spelling.

Tests: 584 PHPUnit on PHP 8.2 and 8.4, 35 node, test:editor and
test:browser green (test:browser reports only the known editor colour
contrast on /grundlagen).

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/EventSubscriber/MaintenanceSubscriber.php

@@ -35,7 +35,7 @@ final class MaintenanceSubscriber implements EventSubscriberInterface
             return;
         }
         $path = $event->getRequest()->getPathInfo();
-        if (preg_match('#^/('.preg_quote($this->adminPath, '#').'|assets/|media/|favicon)#', $path)) {
+        if (preg_match('#^/('.preg_quote($this->adminPath, '#').'|assets/|media/|favicon|\.well-known/kantsites-origin)#', $path)) {
             return;
         }
         $since = @file_get_contents($this->projectDir.'/'.Updater::MAINTENANCE_FILE);