China placed all 17 million residents in one of its biggest cities under lockdown on Sunday, as virus cases doubled nationwide to nearly 3,400 and anxiety mounted over the resilience of its ‘zero-Covid’ approach in the face of the worst outbreak in two years. The southern tech hub of Shenzhen told all residents to stay at home as the city struggles to eradicate an Omicron flare-up linked to neighbouring virus-ravaged Hong Kong. The lockdown and a suspension of public transport will last until 20 March, a city government notice said, adding that it would launch three rounds of mass testing. The…