VILNIUS, Lithuania — The cafes and bars in Vilnius, Lithuania’s capital, bustle with fashionably dressed 20-somethings listening to music, sipping mango matchas and drinking local craft beer — what you’d expect in one of the most vibrant economies in Europe and a place recently pegged as having “the world’s happiest young people.”Thirty-six years after becoming the first Soviet republic to declare independence and more than two decades after joining the European Union, Lithuania is home to six tech unicorns, startups valued at more than $1 billion. It’s also a place where as prosperity has risen, voter participation has dropped.But over...