fix(dashboard): remove lastSeen from metrics useMemo deps to break render loop

lastSeen tracked workspacesQ.dataUpdatedAt which updates on every background
refetch, creating a new metrics object reference each cycle. The useEffect
dep on metrics then fired unconditionally, calling onMetrics → setMetricsById
→ Dashboard re-render → another refetch → infinite loop.

computeFleetAggregates never reads lastSeen, so reporting it upward was
pointless. Hardcode null and drop it from the dep array entirely.

packages/web/src/components/dashboard/ServerWorkspaceRows.tsx

@@ -88,10 +88,10 @@ export function ServerWorkspaceRows({ instance, onOpenWorkspace, onMetrics }: Pr
 			conclusionCount,
 			queueActive,
 			queuePending,
-			lastSeen,
+			lastSeen: null,
 			health,
 		}),
-		[totalWorkspaces, conclusionCount, queueActive, queuePending, lastSeen, health],
+		[totalWorkspaces, conclusionCount, queueActive, queuePending, health],
 	);
 
 	const lastReported = useRef<FleetRowMetrics | null>(null);