Plummeting oil prices and the failure so far to conclude a nuclear agreement are dealing a double blow to Iran’s economy just as it was starting to recover. Oil prices, which have fallen about 50% since June, resumed their decline on Monday after Saudi Arabia, the world’s largest crude exporter, said it may boost production despite a global glut. The slide in prices is strangling revenues in Iran, a country dependent on oil money. Compounding the problem, Iran and six world powers also missed a deadline in November to conclude a comprehensive deal on limiting Iran’s nuclear activities, something that has dimmed hopes for crippling economic sanctions to be lifted anytime soon. The renewed economic travails are eroding gains that accompanied President Hasan Rouhani ’s surprise election 18 months ago. Mr. Rouhani, whose political fortunes rest largely on an economic recovery, has been scrambling to contain the damage. […]