fix: guard systemConfigAtom reads against null

``logout`` calls ``clearAtomStorage(systemConfigAtom)`` which sets the
atom to ``null``, and on a fresh SPA login (no full reload)
``app.tsx``'s ``fetchSystemConfig`` doesn't re-run, so consumers that
read ``systemConfig.xxx`` directly crash with "Cannot read properties
of null". A hard refresh masks the bug. Add optional chaining at the
read sites (grafana url / dashboard uids, showMonitoring) so the
contract stays robust regardless of how the atom is reset.

src/pages/llmodels/hooks/use-models-columns.tsx

@@ -127,7 +127,7 @@ const useModelsColumns = ({
         return record.replicas > 0;
       }
       if (action.key === 'metrics') {
-        return systemConfig.showMonitoring;
+        return systemConfig?.showMonitoring;
       }
 
       return true;