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/schemas/client.ts

@@ -112,6 +112,11 @@ export const BulkDetachResultSchema = z.object({
 
 export const OnlinesSchema = nullableStringArray;
 
+export const OnlineByNodeSchema = z
+  .record(z.string(), nullableStringArray)
+  .nullable()
+  .transform((v) => v ?? {});
+
 export const GroupSummarySchema = z.object({
   name: z.string(),
   clientCount: z.number(),