“That is a crotch shot!” says Cheng Lei, laughing at my phone’s screensaver. I tell her it’s a picture of my legs but she’s sceptical and laughs even more. You could be forgiven for expecting Cheng, 49, to be serious and sombre: the journalist spent two years and three months in prison in China for an absurd crime. Detained as a spy and kept in isolation for months, she was constantly watched by guards, and did not hear the voices of her two children for years. But she displays a…