JPMorgan gained three years' worth of new clients in a single weekend when Silicon Valley Bank collapsed. The Jamie Dimon-led bank has since quadrupled its startup client base to nearly 12,000, served by 550 bankers. JPMorgan's goal is to be the one-stop shop for founders from seed round to IPO and beyond. People line up outside of the shuttered Silicon Valley Bank (SVB) headquarters on March 10, 2023 in Santa Clara, California. Justin Sullivan | Getty Images Three years ago, JPMorgan Chase executive Doug Petno was at a New York City party celebrating a colleague's retirement when his boss, Jamie…