Betty Tamm cranes her neck and points up to a spot on the pale blue stucco exterior of the downtown building."There used to be a great big sign E You can see one of the brackets there. It was way, way up, and it was a big, tall sign," Tamm says of the former Grand Hotel, 740 Cass Ave.If all goes according to plan, sounds of life -- and another vertical sign, saying Grand Apartments -- will return to the building's upper floors within two years. The historic five-story landmark, once a hub of downtown activity, has been mostly vacant …