BEIJING — For President Donald Trump, the first day of his visit to Beijing was all about the personal relationship between him and Xi Jinping, the Chinese leader. “You’re a great leader,” he told his host, whom he has often said he admires for his “powerful” control over a nation of 1.4 billion people. “I say it to everybody.” Xi, unsurprisingly, spent little time Thursday on flattery. Once the 21-gun salute and precision marching by units of the People’s Liberation Army were finished, the disciplined Chinese leader plunged right away into setting boundaries for the two countries’ relations. The red line was Taiwan, he said, making it abundantly clear that Trump’s effort at rapprochement could crash on takeoff if he interferes with China’s long-term effort to take control of the self-governing island. “The U.S. must handle the Taiwan issue with utmost caution,” he said according to a readout from Xinhua, China’s official news agency.…