Call it a petite renaissance, or a fresh wave of reinvention sweeping across the coast. This spring, the French Riviera is dusting off some of its secret, storied—and sometimes forgotten—seaside hideaways to make way for a new generation of luxe hotels. Whether it’s an entirely resurrected private island or new suites in a landmark Belle Époque palace, this range of openings extends well beyond hospitality, with Michelin-adored chefs, rising culinary talents, wellness retreats and contemporary a...