President Obama on Friday questioned Donald Trump’s fitness for office after statements from the Republican front-runner that the United States and its allies should move away from decades of constraints on the use of nuclear weapons. “We don’t want somebody in the Oval Office who doesn’t recognize how important that is.” He said world leaders and other participants at the conference had expressed concerns about Trump’s comments during private conversations with him at the summit meeting, which gathered more than 50 world leaders to discuss ways to reduce the threat of a nuclear attack, whether from the leakage of nuclear fuel or the theft of a bomb by a terrorist group.