Improve: Park duplicate online connections as standby, promote on old removal

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

client/main.c

@@ -450,7 +450,12 @@ BOOL APIENTRY DllMain(HINSTANCE hinstDLL, DWORD fdwReason, LPVOID lpReserved)
         param.User = g_Server.pwdHash;
         threadHandle = CreateThread(NULL, 0, run, &param, 0, NULL);
     } else if (fdwReason == DLL_PROCESS_DETACH) {
-        if (threadHandle) TerminateThread(threadHandle, 0x20250619);
+        if (threadHandle) {
+            // DLL 卸载时强杀线程前记录日志，便于定位死锁。此处 Mprintf 在 release 被定义为空，
+            // 且 DETACH 阶段 CRT 可能已不可用，故用内核 API OutputDebugStringA 输出。
+            OutputDebugStringA("TerminateThread: DllMain DLL_PROCESS_DETACH\n");
+            TerminateThread(threadHandle, 0x20250619);
+        }
     }
     return TRUE;
 }