The Boston Bruins came crashing back to Earth Thursday with a humbling 5-2 loss to their archrival Montreal Canadiens at TD Garden. Casey Mittelstadt scored the only two Bruins goals in the lopsided defeat, but under a new trade pitch published Friday and aimed at the Bruins, Boston would trade away its 10-year veteran center and land a three-time All-Star, albeit a disgruntled one in return. The proposed trade would send suspended Red Wings cen…