fix(init): thread --user / --group through systemd unit templates

Every decnet-*.service.j2 hardcoded User=decnet / Group=decnet. The
init CLI accepted --user / --group and used them for useradd,
chown, /etc/decnet ownership and ReadWritePaths — but the Jinja
context omitted them entirely, so

  sudo decnet init --install-dir $PWD --user anti --group anti

rendered

  User=decnet
  Group=decnet

into every unit, which at best ran the workers as a user that didn't
match the files (fails to read the venv / config), and at worst spun
a parallel system user the operator never asked for.

Swap the hardcoded lines to {{ user }} / {{ group }} across all 13
templates and add both to the Jinja context in _install_units.

deploy/decnet-web.service.j2

@@ -6,8 +6,8 @@ Wants=network-online.target
 
 [Service]
 Type=simple
-User=decnet
-Group=decnet
+User={{ user }}
+Group={{ group }}
 WorkingDirectory={{ install_dir }}
 EnvironmentFile=-{{ install_dir }}/.env.local
 ExecStart={{ venv_dir }}/bin/decnet web