HONG KONG—Hong Kong authorities will set up polling stations at the border with mainland China to allow Hong Kongers living there to vote during upcoming legislative elections, the city’s leader said Tuesday. During the Dec. 19 legislative elections, registered Hong Kong permanent residents living in the mainland will be able to cross the border into the city to vote at the special polling stations, Carrie Lam said. After voting, they must return immediately and will not need to undergo coronavirus quarantines in mainland China, Lam said at a regular news conference. “Currently, there are still people stranded on the mainland, they can’t come back to work or study, and they won’t be able to come back to Hong Kong to vote [without these measures],” she said. Currently, those who travel from Hong Kong to mainland China must undergo at least 14 days of quarantine. Travelers from mainland China to Hong …