When services.syncthing.guiAddress is a unix socket, curlAddressArgs built
`--unix-socket <sock> http://.<path>`. curl >= 8.21 tightened URL parsing and
rejects the bare-dot authority with "URL rejected: Bad hostname" (exit 3), so
every REST call in the merge script fails. Combined with `--retry 1000
--retry-delay 1 --retry-all-errors`, syncthing-init.service retries until
systemd terminates it at the start timeout and Home Manager activation fails.
Use `localhost` as the placeholder authority; --unix-socket still routes the
request to the socket, and localhost is accepted by all curl versions. Add a
regression test asserting the unix-socket path uses http://localhost.
