Relaunch through UAC when (un)installing the service unprivileged

--install-service/--remove-service on Windows no longer fail with
'Access is denied' from a normal shell: the process re-runs itself via
ShellExecuteEx 'runas', waits for the elevated child and mirrors its
exit code. The child gets --elevated-child and pauses for a keypress so
its console output stays readable. Declining the prompt reports
'UAC prompt declined'.

internal/service/service_linux.go

@@ -33,6 +33,15 @@ const (
 // IsService reports whether the process was started by systemd.
 func IsService() bool { return os.Getenv("INVOCATION_ID") != "" }
 
+// Elevated is always true on Linux: there is no UAC equivalent; privilege
+// errors surface from the failing operation with a "run as root" hint.
+func Elevated() bool { return true }
+
+// RelaunchElevated is a Windows-only concept (UAC).
+func RelaunchElevated([]string) (int, error) {
+	return 0, fmt.Errorf("elevated relaunch is only supported on Windows")
+}
+
 // Run executes run with SIGINT/SIGTERM cancellation (which is how systemctl
 // stop signals the process) and tells systemd when the shutdown begins.
 func Run(run func(ctx context.Context) error) error {