Each year, more than 500,000 people are witness to the largest annual fireworks display in America. It’s the beginning of the Kentucky Derby Festival and the unofficial start of springtime in the city. The 28-minute fireworks show, Thunder Over Louisville, launches in a suite on the 24th Floor of the Galt House, where a command center coordinates everything from the soundtrack to traffic on the Ohio River. This is the 25th year and, with good weather expected, maybe the best yet.…