Set comfy backend host/port from serve.py

[halr9000]

Similar to #97 . I'd like this as I'm running multiple comfyui instances on the same host with different configurations -- and I am a heavy mobile user. This works ok now, but not great:

• annoying to have to go into settings to change the backend settings from each browser
• the mobile site doesn't seem to let you change the backend at all. It just gives a fatal error at the /mobile endpoint. Tried to request desktop site but it didn't help

Yeah you could just fix the mobile backend setting, but it'd still be nice for box to come up correctly each time based on start arguments.

P.S. I love your project keep it up