frontend: hide radio-bot pool controls from non-admins (server-26#213 item 4)

GET /tokens and PUT /tokens/{id}/prefer became platform-admin only in #216
(server-26#204 item 6). Three frontend surfaces still offered them to
operators:

- systems page "Preferred Bot Token" panel: now rendered for admins only.
  No non-admin listing endpoint was added: the write behind the picker is
  admin-only too, so a list would only feed a control that 403s on save.
  The panel also no longer turns a fetch error into "No tokens in pool."
- /tokens page: admin-only (operators are sent to /network).
- /network hub: the Bot Tokens card is shown to admins only.

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

drb-frontend/app/tokens/page.tsx

@@ -15,7 +15,9 @@ interface TokenRecord {
 }
 
 export default function TokensPage() {
-  const { isAdmin, isOperator, loading: authLoading } = useAuth();
+  // Platform admin only: every /tokens route is require_admin_token since
+  // server-26#204 item 6, so an operator here only ever saw a failed fetch.
+  const { isAdmin, loading: authLoading } = useAuth();
   const router = useRouter();
   const [tokens, setTokens] = useState<TokenRecord[]>([]);
   const [loading, setLoading] = useState(true);
@@ -26,8 +28,8 @@ export default function TokensPage() {
   const [error, setError] = useState<string | null>(null);
 
   useEffect(() => {
-    if (!authLoading && !isAdmin && !isOperator) router.replace("/");
-  }, [authLoading, isAdmin, isOperator, router]);
+    if (!authLoading && !isAdmin) router.replace("/network");
+  }, [authLoading, isAdmin, router]);
 
   const refresh = useCallback(async () => {
     try {
@@ -38,7 +40,7 @@ export default function TokensPage() {
     }
   }, []);
 
-  useEffect(() => { refresh(); }, [refresh]);
+  useEffect(() => { if (isAdmin) refresh(); }, [isAdmin, refresh]);
 
   async function handleAdd(e: React.FormEvent) {
     e.preventDefault();
@@ -67,7 +69,7 @@ export default function TokensPage() {
     }
   }
 
-  if (authLoading || (!isAdmin && !isOperator)) return null;
+  if (authLoading || !isAdmin) return null;
 
   return (
     <div className="space-y-6">