They banged on windows, screamed for help, dropped children from smoky floors in a desperate attempt to save them. Terrified residents of the Grenfell Tower said there was little warning of the inferno that engulfed their high-rise apartment building and left 12 people dead — a toll that officials said would almost certainly rise. The blaze early Wednesday in the 24-story building in west London’s North Kensington district also injured 74 others, 18 of them critically, and left an unknown number missing. A tenants’ group had complained for years about the risk of a fire.