Replacing the old connection on a duplicate login made the client
reconnect in a tight loop. Instead, park the new connection in a standby
list keyed by clientID and promote it into the main host list only after
the old connection is removed by heartbeat timeout or disconnect.
Fix heartbeat attribution so a parked standby refreshes its own
last-heartbeat time rather than the main connection's, otherwise the main
never times out and the standby can never take over. Gate the offline
notification on an actual removal with no promoted standby to avoid a
spurious "host offline" when takeover happens. Skip promoting a standby
already marked removed, and reuse the normal login sign/settings path when
a standby is promoted.
Co-Authored-By: deepseek-v4-pro
