Support different local port

Currently, the extension only sends data to port 5600.

I have two activitywatch servers running on my system for two different users (on port 5600 and port 5700)

And I want this watcher to send my data to port 5700!

It was mentioned in [this comment](https://github.com/ActivityWatch/aw-watcher-web/issues/44#issuecomment-592902059) that editing the IP address will not be supported, which is understandable!

But, can we add support for editing local port? That doesn't seem too big of a security threat to me.