WASHINGTON — President Joe Biden on Monday took fresh aim at ghost guns, the privately made firearms without serial numbers that are increasingly cropping up in violent crimes, as he struggles to break past gun-control opposition to address firearm deaths.Speaking at the White House, Biden highlighted the Justice Department’s work to finalize new regulations to crack down on ghost guns, and announced the nomination of Steve Dettelbach, who served as a U.S. attorney in Ohio from 2009 to 2016, to run the Bureau …