It’s cramped, it’s colorful, it’s overwhelming: it’s the North End fit-to-burst with life on a sunny day. The hub of Boston’s Little Italy is the “top” of Hanover Street at its intersection with Richmond and Parmenter. This is where Quattro — which took over the Il Panino Express spot this spring — stands, inviting passerbys into its cool interior of sleek pale marble countertops, wall-to-wall white, and floor-to-ceiling windows that connect it to the bustle of the street when weather permits.