The Chinese singer stands on a balcony inside a bombed-out theater in the Ukrainian city of Mariupol, the site of a deadly attack last year by Russian forces. Looking at the camera, she sings an excerpt from the Soviet-era patriotic song “Katyusha” and lifts her arms triumphantly into the air. The video of the singer, Wang Fang, a 38-year-old performer of patriotic songs and Chinese opera, has circulated widely online in recent days, fueling outrage in Ukraine and abroad. She appeared in Mariupol last week as part of a visit…