fix(systemd): include service user in service file generation

* Add ServiceUser field to Config struct for user specification.
* Update Generate and Install functions to accept service user.
* Modify tests to reflect changes in service user handling.

systemd/generator.go

@@ -17,6 +17,7 @@ After=network.target
 
 [Service]
 Type=simple
+User={{.ServiceUser}}
 ExecStart={{.ExecStart}}
 ExecStop={{.ExecStop}}
 Restart=on-failure
@@ -49,6 +50,7 @@ type templateData struct {
 	Unit        config.Unit
 	ExecStart   string
 	ExecStop    string
+	ServiceUser string
 }
 
 // ServiceName returns the systemd service name for a unit, with optional prefix/suffix.
@@ -57,7 +59,7 @@ func ServiceName(u config.Unit, prefix, suffix string) string {
 }
 
 // Generate produces the .service file content for a unit.
-func Generate(u config.Unit, prefix, suffix string) (string, error) {
+func Generate(u config.Unit, prefix, suffix, serviceUser string) (string, error) {
 	execStart, execStop := buildExecCommands(u)
 
 	data := templateData{
@@ -65,6 +67,7 @@ func Generate(u config.Unit, prefix, suffix string) (string, error) {
 		Unit:        u,
 		ExecStart:   execStart,
 		ExecStop:    execStop,
+		ServiceUser: serviceUser,
 	}
 
 	tmplStr := systemUnitTemplate