HARARE, Zimbabwe — Zimbabwe imposed a “total internet shutdown” for most of Friday in what critics called an attempt to hide growing reports of a violent crackdown on protests against a dramatic fuel price increase. Badly injured people streamed into a hospital in the capital, and rights groups said at least 12 people have been killed this week. Gasoline in the economically shattered country is now the world’s most expensive, and since Monday Zimbabweans have heeded a nationwide call shared on social media to stay at home in protest.