The swanky interior of one of United Airlines’ newest Boeing 787-9s is a lodestar for the future of U.S. aviation — and a throwback to a bygone era. Two-thirds of the 206-foot-long airplane is earmarked for premium seating. You won’t see an economy class seat until after the aircraft’s third set of exit doors behind the wing. It’s a layout reminiscent of the start of the jet age almost seven…