With U.S. union ranks swelling as everyone from coffee shop baristas to warehouse workers seeks to organize, Illinois voters will decide next month whether to amend their state constitution to guarantee the right to bargain collectively.The fate of the ballot measure is being closely watched in Illinois and beyond, as it will gauge public support for the labor movement that has lost ground for years in conservative-led states. Unions and pro-industry groups say it could signal a new chapter in the struggle over workers …