The muted mood of Hunter Biden’s trial, which had focused on his struggle with addiction, shifted abruptly on Wednesday, with the appearance of Zoe Kestan, a former girlfriend who described him as a charming free-spender who loved the party-hopping high life in New York. Ms. Kestan, a dancer Mr. Biden met at a gentleman’s club in 2018, said the two immediately connected — “catching feelings” as she put it — after she sat with him in a quiet back room and clicked on a song from Fleet Foxes,...