A full-floor dwelling at the prestigious 845 Fifth Ave. co-op is on sale for the first time in nearly 60 years, looking nearly untouched from that long-ago era. The $30 million listing encompasses the entire second floor of the 17-unit building, plus a guest suite on the ground floor. But even better are its eye-catching interiors, which look frozen in amber. The listing, held by Emanuele Fiore of Serhant, advertises a “mint, museum-quality condition,” with modern renovations whose listing