Though visibly asleep during most of the congress, Mr. Mugabe, the world’s oldest head of state and the only leader Zimbabwe has known since its independence in 1980, was selected as his party’s candidate in the 2018 presidential election. At the congress, Mr. Mugabe appeared increasingly dependent on his wife, 51. When a waiter carrying bags of potato chips on a silver tray startled Mr. Mugabe, the first lady chose a bag from which the president then slowly picked out one single chip after another.