fix: auto-refresh enrolled devices list every 30 seconds

The useGpsDevices query was missing refetchInterval, so the Last Active
timestamp on the Enrolled Devices page only updated on initial page load.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/hooks/useGps.ts

@@ -74,6 +74,7 @@ export function useGpsDevices() {
       const { data } = await api.get('/gps/devices');
       return data;
     },
+    refetchInterval: 30000, // Refresh every 30 seconds to update lastActive
   });
 }