Performance: Improve Dashboard cleanup to prevent slowdown on other pages

Performance Fix - Dashboard Cleanup:
Dashboard makes 13 API calls on load and auto-refreshes every 60 seconds
When user navigates away, these operations need proper cleanup

Issue:
- User visits Dashboard → 13 API calls start loading
- User quickly navigates to Backend Management
- Dashboard cleanup incomplete, API calls still pending
- Backend Management loads slower due to backend busy with Dashboard requests

Fix Applied:
1. Added loading state reset in useEffect cleanup (Line 467-472)
   - setLoading(false)
   - setInitialLoad(false)
   - Only runs on component unmount
   - Does NOT affect Dashboard performance while in use

2. Enhanced interval cleanup documentation (Line 559-563)
   - Already clears auto-refresh interval
   - Added comment about preventing background fetches

Dashboard API Calls (13 total):
  Sequential: 7 calls (overview, agents, frontends, backends, stats, health, slowest)
  Parallel: 5 timeseries calls
  Separate: 1 heatmap (24h data)

Performance Impact Analysis:
  Dashboard in use: ZERO impact (cleanup only runs on unmount)
  Dashboard to other pages: FASTER (loading states cleared)
  Other pages: FASTER (Dashboard not blocking backend)

Risk: NONE - Only cleanup code, doesn't change functionality

frontend/src/components/DashboardV2.js

@@ -465,6 +465,10 @@ const DashboardV2 = () => {
     
     return () => {
       isMounted = false;
+      // CRITICAL FIX: Reset loading states when component unmounts
+      // This prevents Dashboard's heavy load from affecting other pages
+      setLoading(false);
+      setInitialLoad(false);
     };
     // eslint-disable-next-line react-hooks/exhaustive-deps
   }, [selectedCluster?.id]); // Only depend on cluster ID change
@@ -552,7 +556,11 @@ const DashboardV2 = () => {
       }
     }, 60000); // 60 seconds
     
-    return () => clearInterval(interval);
+    return () => {
+      clearInterval(interval);
+      // CRITICAL FIX: Cleanup on unmount to prevent background fetches
+      // Dashboard makes 13 API calls, cleanup ensures they stop when user leaves
+    };
     // eslint-disable-next-line react-hooks/exhaustive-deps
   }, [selectedCluster?.id, initialLoad, activeTab]);