Russian opposition figure Alexei Navalny’s death Friday at a Russian prison camp in the Arctic silenced a man who was arguably the most influential remaining critic of President Vladimir Putin and the authoritarian state the former spy has methodically built on the wreckage of the Soviet Union. Putin, who has effectively run Russia for 24 years and is seeking to extend his time in office for another six years in elections set for next month, now strides the Russian political stage with almost no visible challengers. Many of those who have opposed him have ended up in prison, or dead....