KATANGA JOHNSON and STEVE HOLLAND PRESIDENT Joe Biden has pledged federal assistance and offered comfort to the families of those killed and missing in last week's Florida condominium collapse after the search-and-rescue operation was suspended due to safety concerns. Biden, whose personal experience with tragedy has marked his political career, traveled to Florida to reprise the role of "consoler-in-chief" a week after the 12-story Champlain Towers South partially caved in as residents slept. The confirmed death toll remained at 18 after the discovery of six more bodies in the ruins of the condo, including two children, aged 4 and 10.…