Package a systemd user service

The flake package supplies a user unit that runs its own daemon binary
at login, restarts failures after five seconds, and sends output to the
journal. SIGTERM starts cleanup, with two minutes before a forced stop.
The unit keeps the rights of the user and a private file creation mask.

A unit in the existing package is the smallest flake interface for this
service. It needs no extra module interface or dependency and leaves the
data directory and tool environment in a standard user drop-in.

The README covers the data copy with the daemon stopped, the shared
client data setting, the package result link, and service controls. A
flake check verifies the built unit without a running user manager.

Issue: 99a5e07