Alexandra de Garidel, an interior designer in Geneva, Switzerland, with a portfolio of lavish projects, made no bones about the stardust in her eyes when she searched for a pied-à-terre in Saint-Germain-des-Prés, a Left Bank district of elegant stone buildings, literary cafes, bakeries and cheese shops. “You want to live in a dreamy Paris, you want to feel like a romantic poet,” said Ms. de Garidel, the founder of a company called Avilda.