BRAINTREE, Mass. — A 53-year-old Massachusetts hospital worker stepped forward Thursday to claim the biggest undivided lottery jackpot in U.S. history — $758.7 million — after breaking the news to her employer the way the rest of us only dream of: “I called and told them I will not be coming back.”“The first thing I want to do is just sit back and relax,” Mavis L. Wanczyk told reporters at a news conference.Wanczyk chose to take a lump-sum payment of $480 million, or $336 million after taxes, lottery officials said. Winners who take a gradual payout stand to get more money spread out over several decades.The previous evening, she recalled, she was leaving work with a firefighter and remarked, “It’s never going to be me. It’s just a pipe dream that I’ve always had.”Then she read the number on her ticket and realized she had won.Wanczyk has two adult children, a daughter and a son.The jackpot is the second-largest U.S. lottery prize, ahead of a $656 million Mega Millions prize won by three people in 2012. But Wednesday’s big prize is still dwarfed by a $1.6 billion Powerball jackpot divvied up between three winners in January 2016.The announcement that a winner had come forward came after a turbulent morning in which lottery officials initially misidentified not only the store that sold the winning ticket, but the town.