A few months after her husband took office, Jill Biden posted a photo of herself sitting at a desk aboard Air Force One on social media looking very important. “Prepping for the G7,” she wrote on her @FLOTUS “US Government Official” account, under the photo in which a scholarly “Dr. Jill” wearing reading glasses, pen in hand, is studying a thick ring binder of notes. A flight jacket with the presidential seal is draped over the back of the chair, to leave you in no doubt about the power behind the throne. As the 72-year-old wife of the oldest president...