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/pages/api-docs/endpoints.ts

@@ -666,9 +666,15 @@ export const sections: readonly Section[] = [
       {
         method: 'POST',
         path: '/panel/api/clients/onlines',
-        summary: 'List the emails of currently connected clients (last seen within the heartbeat window).',
+        summary: 'List the emails of currently connected clients (last seen within the heartbeat window), deduped across every node.',
         response: '{\n  "success": true,\n  "obj": ["user1", "user2"]\n}',
       },
+      {
+        method: 'POST',
+        path: '/panel/api/clients/onlinesByNode',
+        summary: 'Online client emails grouped by the node that reported them. The local panel uses key "0"; each remote node uses its node id. Lets the inbounds page show online status per node instead of merging every node together.',
+        response: '{\n  "success": true,\n  "obj": {\n    "0": ["user1"],\n    "3": ["user1", "user2"]\n  }\n}',
+      },
       {
         method: 'POST',
         path: '/panel/api/clients/lastOnline',