Russian opposition leader Alexei Navalny died in prison after being poisoned with a deadly toxin found in Ecuadorian dart frogs, the U.K. and other allies claimed Saturday. There is no possible explanation other than a deliberate poisoning, European officials said, noting that the frogs are not found in Russia. Russia’s prison service reported in February 2024 that Navalny died after having felt unwell following a walk around the high-security facility in a remote town above the Arctic Circle where he was serving a combined 30½-year jail sentence. He was 47. “The U.K., Sweden, France, Germany and the Netherlands are confident...