Richard Dobatse, a Navy medic in San Diego, dabbled infrequently in stock trading. But his behavior changed in 2017 when he signed up for Robinhood, a trading app that made buying and selling stocks simple and seemingly free. Dobatse, now 32, said he had been charmed by Robinhood’s one-click trading, easy access to complex investment products, and features like falling confetti and emoji-filled phone notifications that made it feel like a game.