Known for its seating, case goods, lighting, outdoor furniture, and other home furnishings that put a modern twist on traditional Italian designs, Fendi Casa is thinking outside the showroom by furnishing model or for-sale units at premier real estate developments. In New York, the private residences at 400 Fifth Avenue (www.400fifthavenue.com) feature the brand’s leather sofas and chairs, lacquer furniture, rugs, and chandeliers in the development’s 3,590-square-foot penthouse, which is priced at about $18 mi…