Russian troops Friday seized the biggest nuclear power plant in Europe after a middle-of-the-night attack that set it on fire and briefly raised worldwide fears of a catastrophe in the most chilling turn in Moscow’s invasion of Ukraine yet. Firefighters put out the blaze, and no radiation was released, U.N. and Ukrainian officials said, as Russian forces pressed on with their week-old offensive on multiple fronts and the number of refugees fleeing the country topped 1.2 million.