The tray Quit menu now disconnects the daemon before exiting instead of
only tearing down the GUI. A new DownAsync RPC lets the daemon start the
teardown and return immediately: beginDown cancels the connection under
the mutex (so it cannot reconnect), then finishDown (the retry-goroutine
wait and status reset) runs on a background goroutine. handleQuit aborts
any in-flight profile switch first (so a queued Up cannot reconnect during
teardown) and calls DownAsync so quitting never blocks on the engine
shutdown.
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