Starbucks is threatening to sue a Vancouver barista who says she was fired for trying to unionize the company’s stores. The coffee giant accuses Frédérique Martineau, 21, and the United Steelworkers union of defaming the company by claiming Martineau was terminated as reprisal for her role in unionizing a now-closed store on Vancouver’s west side. In a Nov. 24 letter obtained by The Tyee, a lawyer representing Starbucks wrote that Martineau and the union had misrepresented the real reasons for h