Fix missing SSH username on initial sync

[QuakeMatt]

Been playing around with this library as part of a Docker workflow, but ran into a problem where the remote username config isn't respected on the initial sync. Seems like it's not written properly to the ssh:// URI in the Unison project file?

Update: Made a small tweak to have it fall back to the OS user when empty, as per the watch connection made in main.ts.