For a few hours on Sunday, Ariana Grande, a 23-year-old pop star from Boca Raton, Florida, was the leader of the free world. The position has been open for months. Contestants ranging from German Chancellor Angela Merkel to, improbably, Chinese President Xi Jinping have been auditioning for the job. [...] While President Trump gutter-tweeted argle-bargle and played another round of golf, Grande delivered what will likely stand as the official American response to the bombing in Manchester and to another terrorist attack, the night before the concert, in London.