fix: run port check before loading custom nodes

Move port availability check to the beginning of main.py, before
custom node loading starts. This prevents users from waiting through
slow custom node initialization only to discover the port is taken.

Closes #8935

main.py

@@ -459,6 +459,19 @@ def start_comfyui(asyncio_loop=None):
 
 
 if __name__ == "__main__":
+    # Early port check before loading custom nodes (which can be slow)
+    import socket
+    try:
+        test_socket = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
+        test_socket.settimeout(1)
+        result = test_socket.connect_ex(('127.0.0.1', args.port))
+        test_socket.close()
+        if result == 0:
+            logging.error("Port {} is already in use. Please specify a different port with --port or stop the process using this port.".format(args.port))
+            sys.exit(1)
+    except OSError as e:
+        logging.warning("Could not check port {}: {}".format(args.port, e))
+
     # Running directly, just start ComfyUI.
     logging.info("Python version: {}".format(sys.version))
     logging.info("ComfyUI version: {}".format(comfyui_version.__version__))