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'.

SPEC.md

@@ -170,9 +170,11 @@ Linux with systemd (the future GPU server), as well as in Docker.
 
 Service management is the same on both platforms:
 `gpu-turnstile --install-service [-config path]` installs, registers and
-starts an auto-start service; `--remove-service` stops and uninstalls it
-(both need an elevated/root shell). The legacy form `gpu-turnstile service
-install|remove` does the same thing.
+starts an auto-start service; `--remove-service` stops and uninstalls it.
+Both need admin/root; on Windows a non-elevated shell triggers a UAC
+prompt instead of failing — the command relaunches itself elevated, waits
+for the child, and mirrors its exit code. The legacy form
+`gpu-turnstile service install|remove` does the same thing.
 
 By default install creates the canonical layout and copies the binary into
 it (Windows: `%ProgramFiles%\gpu-turnstile\`, plus