BOSTON — Francis Salemme leaned forward in his chair and craned his neck to get a better look at the photo being shown to jurors. On the computer screen was Salemme with brown curly hair, at the time the New England Mafia boss known as “Cadillac Frank,” holding court outside a hotel under the watchful eye of the FBI. Almost three decades later, the 84-year-old Salemme, on trial in the strangling of a nightclub owner in 1993, would fit in better at a nursing home than at the helm of a Mafia meeting. For prosecutors, Salemme is perhaps the last man standing from an era when organized crime flourished in Boston and its environs.