DUBAI, United Arab Emirates — Iran and the United States were at an impasse again Monday over how to end their war while their ceasefire grew increasingly shaky, with the two sides exchanging fire in recent days, ships and Gulf states being targeted, and fighting flaring between Israel and the Lebanese militant group Hezbollah. The volatility could tip the Middle East back into open warfare and prolong the worldwide energy crisis sparked by the conflict, with Iran’s chokehold on the Strait of Hormuz and America’s blockade of Iranian ports still in place. U.S. President Donald Trump is expected to use a trip this week to China to urge Chinese President Xi Jinping to pressure Iran into making concessions and end the limbo. Beijing is the biggest buyer of the Islamic Republic’s sanctioned crude oil, giving it leverage. But getting to any deal likely remains tough work.…