The Hayden Building, the last commercial building in Boston designed by the famed Henry Hobson Richardson, architect of Trinity Church, is being renovated after decades of decline and neglect. The 1875 building, which sat squarely in what became the Combat Zone, Boston’s now-faded and rapidly redeveloping red light district, had been a shell of itself since a 1985 fire that evicted the peep show and gay bathhouse occupying it at the time. The preservation group Historic Boston is leading a $5.8 million renovation that will yield apartments and retail space.