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

frontend/src/api/queryKeys.ts

@@ -21,6 +21,7 @@ export const keys = {
     list: (params: unknown) => ['clients', 'list', params] as const,
     all: () => ['clients', 'all'] as const,
     onlines: () => ['clients', 'onlines'] as const,
+    onlinesByNode: () => ['clients', 'onlinesByNode'] as const,
     lastOnline: () => ['clients', 'lastOnline'] as const,
     groups: () => ['clients', 'groups'] as const,
   },