On Monday nights, the Beech Street Center comes alive with thumping beats, shouts, and laughter galore. A small — but mighty — cohort of Belmont residents gathers each week for 45 minutes of rigorous Zumba class. “Dance is magical. It builds community, it builds confidence. It’s the best,” said Ann Callahan, who runs the weekly sessions. Founded in the 1990s, Zumba is a popular aerobic fitness program that combines music and dance moves. Callahan has been teaching Zumba classes for…