SEOUL, South Korea — Kim Jong Un offered China’s president a grand welcome Monday. But the North Korean leader is playing host from a position of rare strength, and his country has come a long way since Xi Jinping’s last visit seven years ago. Kim’s backing of Russia’s war with Ukraine has paid dividends, his weapons program has cemented North Korea’s status as a de facto nuclear state, and an economy that buckled under the pressure of pandemic isolation and sanctions has since rebounded. Kim