NEW YORK — President Donald Trump added economic action to his fiery military threats against North Korea on Thursday, authorizing stiffer new sanctions in response to the Koreans’ nuclear weapons advances. He said China was imposing major banking sanctions, too, but there was no immediate confirmation from the North’s most important trading partner. Trump praised China for instructing its banks to cut off business with Pyongyang, but neither the Chinese nor Trump officials were ready to say so.