realmd: Handle client changes while Leave dialog is open

The client recreates all proxies when the superuser thing emits a
"changed" event. When this happens while the Leave dialog is open, the
"realm" proxy in the Leave dialog becomes unusable.

pkg/systemd/overview-cards/realmd.jsx

@@ -261,6 +261,7 @@ const LeaveDialog = ({ realmd_client }) => {
     const [expanded, setExpanded] = useState(false);
     const [pending, setPending] = useState(false);
     const [error, setError] = useState(null);
+    useEvent(realmd_client, "changed");
     const realm = realmd_client.joined[0];
 
     const onLeave = () => {