Queen Elizabeth II on Sunday brought the curtain down on her historic Platinum Jubilee celebrations, making a rare public appearance on the last of four days of festivities and vowing to maintain her record-breaking reign. The 96-year-old monarch, who has been dogged by problems with walking and standing, appeared for the first time in person since two showings Thursday on the Buckingham Palace balcony after the Trooping the Colour military parade. That effort forced her to pull out of a thanksgiving church service on Friday, plus the Epsom Derby horse race and a star-studded concert on Saturd…