fix(online): scope online status per node instead of a global union

The inbounds page and Nodes page checked each client's email against a
single deduped union of every node's online clients, so a client connected
to one node showed as online on every inbound across every node. The local
online set was also derived from the email-keyed client_traffics.last_online
column, which remote-node syncs bump too, leaking remote-only clients onto
local inbounds.

Track online clients per node: the local panel's own xray clients under key
0 (derived from live traffic-poll deltas via RefreshLocalOnline, kept in
memory and independent of the shared last_online column) and each remote
node under its id. Add GetOnlineClientsByNode plus a /clients/onlinesByNode
endpoint and onlineByNode WS field; node.go and the inbounds rollup now scope
online by node. The flat GetOnlineClients union is kept for client-centric and
total-count views (Clients page, dashboard, telegram).

Closes #4809

web/job/node_traffic_sync_job.go

@@ -109,7 +109,10 @@ func (j *NodeTrafficSyncJob) Run() {
 		lastOnline = map[string]int64{}
 	}
 
-	j.inboundService.RefreshOnlineClientsFromMap(lastOnline)
+	// Prune stale local-online entries (no local active emails to add here —
+	// only the local xray poll feeds those) so a stopped local xray's clients
+	// still age out between traffic polls.
+	j.inboundService.RefreshLocalOnlineClients(nil)
 
 	if !websocket.HasClients() {
 		return
@@ -121,6 +124,7 @@ func (j *NodeTrafficSyncJob) Run() {
 	}
 	websocket.BroadcastTraffic(map[string]any{
 		"onlineClients": online,
+		"onlineByNode":  j.inboundService.GetOnlineClientsByNode(),
 		"lastOnlineMap": lastOnline,
 	})