Inside Banana Republic’s design studio in San Francisco, Sandra Stangl, the company’s chief executive, pointed to an item that had been creating buzz in stores.It was not a shirt or dress draped on a mannequin. Instead, Ms. Stangl walked toward a king-size bed with a parchment-colored backboard. The company has started putting these bed frames, which sell for around $5,000, near the front of its stores in Los Angeles and New York. Enough shoppers have asked if they were for sale — the answer: Not yet, but Ms. Stangl and her team are taking a limited number of pre-orders for the fall.Shoppers usually think about outfitting themselves, not their homes, when they walk int …