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'.

README.md

@@ -97,9 +97,9 @@ go build ./cmd/gpu-turnstile
 ### Run natively on Windows (current primary deployment)
 
 Download `gpu-turnstile.exe` from a release and install it as a Windows
-service from an elevated shell — the installer creates the canonical
-layout, copies the binary and (if none exists yet) your
-`gpu-turnstile.env` into it, and registers the copy:
+service — no admin shell needed, a UAC prompt appears automatically and
+the elevated child does the work (its window waits for Enter so you can
+read the result):
 
 ```sh
 gpu-turnstile.exe --install-service        # installs into Program Files, auto-start