Home-furnishings retailer Ikea plans to add as many as 15 new stores in the U.S. and Canada during the next five years, as it builds North America into its biggest single market. The Swedish company already has purchased land for a Boston store, said Jan Kjellman, president of Ikea North America. On April 12, it will open its 14th U.S. store, a 274,000 square-foot location in the San Francisco Bay Area. Closely held Ikea is opening two stores this year to add to the 20 it already operates in the U.S. and Canada. It has 156 stores in 29 countries.