A remote-SSH session's workingDir is a path on the remote host, and the
image watcher watched it on the local disk: a folder that is not there,
or a same-named local folder whose images then popped up as the remote
session's. Same root cause as the Ralph @fix_plan.md watcher fix.
Three places start a watch, and all three now skip a remote session:
setupSessionListeners(), the app-wide toggle in PUT /api/settings (which
re-watches every enabled session), and POST /api/sessions/:id/image-watcher,
which refuses to turn it on (INVALID_INPUT, like the Git fetch settings
route) and still lets it be turned off. The Session Options row is hidden
for a remote session, the way the Git tab is, since the client's
_apiPost() does not throw on a refused request.
