ActivityWatch doesn't behave well on multiuser systems

Tried starting two instances of ActivityWatch today on the same computer (different users, Windows).

I was surprised to see the other users data when I opened localhost:5600, then I realized that it's not that strange since they use the same port.

What did surprise me was that I got no error about the server being unable to bind the port. I guess things work differently in the Windows world.

So this has a few issues:

- Users could read the data from other users running ActivityWatch.
- Two users cannot run ActivityWatch **on the same port** (well, they can, but only the person first starting the server will store all the data since the second server will crash on start when it can't bind the port)

This issue is present with other software as well, such as Syncthing. **The basic workaround seems to be to just add the ability to run it on different ports.** But this has some privacy issues if we don't also add the ability to password-protect the server.

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