This was supposed to be the summer Sebastian Siemiatkowski hit the jackpot. The founder of Klarna Group Plc, a 20-year-old financial-technology company from Sweden, had planned an initial public offering on the New York Stock Exchange in the spring. Seeking to raise at least $1 billion, the listing would have cemented Siemiatkowski’s status as a billionaire, at least on paper. But as the clock ticked down to the debut, President Donald Trump announced his plans for stiff tariffs, sending global ...