WASHINGTON — The U.S. Department of Defense’s ability to fend off attacks on the electromagnetic spectrum has atrophied over decades, leaving troops vulnerable on the high-tech battlefields of tomorrow, according to President Joe Biden’s pick to be the nation’s highest-ranking military officer. Air Force Chief of Staff Gen. CQ Brown, nominated in May to be the next chairman of the Joint Chiefs of Staff , told senators this week at his confirmation hearing that the military “has lost some muscle