By Paul Carsten BEIJING (Reuters) – Chinese e-commerce giant Alibaba Group Holding Ltd [IPO-ALIB.N] may have dominated online retail on personal computers, but is some way from replicating that leadership in shopping by smartphone and other mobile devices. Alibaba, which is heading towards a bumper New York IPO later this year, is throwing billions of dollars at figuring out how to thrive as half a billion people, 80 percent of Chinas 618 million internet users, go online via mobile. Both problems threaten to squeeze future profitability. Making life tougher, rival Tencent Holdings Ltd has already planted its flag on smartphone screens with WeChat, the nearly ubiquitous app that has gone from a mobile messaging tool to a digital Swiss Army knife.