REYKJAVIK, Iceland — On a chilly afternoon in October, Frida Ros Valdimarsdottir, a former home-care worker turned women’s rights advocate, left her office at exactly 2:38 p.m. and headed to Reykjavik’s main square, where throngs of women were forming a boisterous crowd. The rally was part of a groundswell for income equality that galvanized tens of...