Hong Kong’s move to mandate the use of its Covid-19 contact-tracing app at government buildings will hit homeless residents and other vulnerable groups in need of services hard given that many lack smartphones, an NGO has warned. The new policy , seen as a first step towards placating Beijing’s concerns over the eventual reopening of the border with mainland China, comes as local officials have been under fire for failing to secure a deal despite an improving vaccination rate and months with effectively no local coronavirus cases.…