Downtown San Francisco’s office buildings have been quieted by some of the highest vacancy rates and slowest return-to-office trends in the country. But when walking around the area, what makes it feel still so uninhabited is a different but related phenomenon downstairs from all those empty offices: the vacant ground floor. It’s the windows with their shades tightly drawn, the phantom deli counters visible through dusty glass, the lingering signage for a Verizon store that doesn’t exist anymore