Add preflight port check before loading startup tasks

[seungwoo-ji-03]

Summary

• add a fail-fast port availability check right after startup begins so we exit before loading custom nodes
• provide clear errors for invalid listen addresses or already-bound ports

Testing

• manual: started ComfyUI with default args; server started successfully
• manual: bound another process to 8188, then started ComfyUI; startup logged port-in-use error and exited

Issue

• Fixes Run port check at the beginning of the main.py #8935