[client] Disconnect daemon on GUI quit via async Down

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.

client/ui/services/connection.go

@@ -177,6 +177,17 @@ func (s *Connection) Down(ctx context.Context) error {
 	return nil
 }
 
+func (s *Connection) DownAsync(ctx context.Context) error {
+	cli, err := s.conn.Client()
+	if err != nil {
+		return err
+	}
+	if _, err = cli.DownAsync(ctx, &proto.DownAsyncRequest{}); err != nil {
+		return s.classifyDaemonError(err)
+	}
+	return nil
+}
+
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 // before the platform default.