Enviros are rejoicing after winning a tough five-year battle against a proposal to build a giant salt plant on the pristine shores of a gray-whale breeding ground on Mexico’s Baja Peninsula. Mitsubishi Corp. and the Mexican government announced yesterday that they are scrapping their joint plan for the $100 million facility. The salt plant, which would have been the largest in the world, was planned for an ecologically sensitive area that had been declared a U.N. World Heritage Site; opponents argued that it would have harmed gray whales and more than 300 other animal species. Enviros mounted an international campaign against the plan, galvanizing scientists, artists, celebrities, and concerned citizens around the globe and motivating 700,000 people to send postcards to Mitsubishi opposing the plant.