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.css

@@ -181,3 +181,10 @@ input[type="text"]:focus-visible, input[type="password"]:focus-visible { border-
 
 #liste:focus { outline: none; }
 #liste[aria-busy="true"] { opacity: .6; transition: opacity .15s; }
+
+.reiter { background: var(--flaeche); border-bottom: 1px solid var(--linie); }
+.reiter-innen { max-width: 1180px; margin: 0 auto; padding: 0 16px; list-style: none; display: flex; gap: 4px; overflow-x: auto; }
+.reiter a { display: block; padding: 12px 14px 10px; color: var(--leise); font-weight: 600; text-decoration: none; border-bottom: 3px solid transparent; white-space: nowrap; }
+.reiter a:hover { color: var(--marke); }
+.reiter a[aria-current="page"] { color: var(--marke); border-bottom-color: var(--marke); }
+.knapp { color: var(--warn); font-weight: 600; }