fix: prevent online clients from randomly disappearing from panel UI (#4387)

* fix: prevent online clients from randomly disappearing from panel UI

Online status was determined solely by whether a client transferred
bytes in the current 5-second polling window. The online list was
completely replaced each cycle, so idle-but-connected clients with no
traffic delta in that window were dropped from the UI.

Now online status is computed from lastOnline DB timestamps with a
5-second grace period via RefreshOnlineClientsFromMap(), so clients
remain visible across idle polling windows.

Closes #4384

* fix: extend online client grace period to survive idle poll cycles

The 5s grace period equalled the traffic-poll interval, so a client
whose Xray stats reported a zero delta for one cycle was still dropped
on the very next tick. Bump to 20s (~4 polls) so idle-but-connected
sessions stay visible across momentary counter gaps without lingering
long after a real disconnect.

Refs #4384

---------

Co-authored-by: MHSanaei <ho3ein.sanaei@gmail.com>

web/job/node_traffic_sync_job.go

@@ -87,10 +87,6 @@ func (j *NodeTrafficSyncJob) Run() {
 		return
 	}
 
-	online := j.inboundService.GetOnlineClients()
-	if online == nil {
-		online = []string{}
-	}
 	lastOnline, err := j.inboundService.GetClientsLastOnline()
 	if err != nil {
 		logger.Warning("node traffic sync: get last-online failed:", err)
@@ -98,6 +94,13 @@ func (j *NodeTrafficSyncJob) Run() {
 	if lastOnline == nil {
 		lastOnline = map[string]int64{}
 	}
+
+	j.inboundService.RefreshOnlineClientsFromMap(lastOnline)
+
+	online := j.inboundService.GetOnlineClients()
+	if online == nil {
+		online = []string{}
+	}
 	websocket.BroadcastTraffic(map[string]any{
 		"onlineClients": online,
 		"lastOnlineMap": lastOnline,