Add a KantTresor tab to the central registry

KantTresor instances (from 1.82.0) report to /api/kanttresor/melden with the
same key-then-HMAC protocol. The new tab lists them with tenants, disk usage
and failing services; it never offers or accepts an update order, since
KantTresor is updated with ./update.sh on the server. See ADR 124.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

zentrale/public/app.js

@@ -98,7 +98,7 @@
     feld.value = '';
     const filter = filterFeld();
     if (filter) filter.remove();
-    laden(new URL('/', location.href), 'push');
+    laden(new URL(form.action, location.href), 'push');
     feld.focus();
   });