An up-and-coming fashion designer who dressed Lady Gaga and other celebrities was murdered in her Manhattan apartment with a lethal cocktail of fentanyl and other drugs last year, cops and reports said Friday. A neighbor spotted Kathryn “Katie” Gallagher’s keys dangling from the door of her apartment on Eldridge Street on the Lower East Side and entered to check on her — then discovered her lifeless body face up on her bed, a police source said. The mysterious death of Gallagher, 35, was classified as a homicide Friday after the medical examiner determined that she died from acute intoxication from...