The Princess of Wales gaily joined the royal family on the balcony of Buckingham Palace Saturday for the King’s official birthday in her first public appearance since her cancer diagnosis. Kate Middleton, husband William, and their children, George, 10, Charlotte, 9, and 6-year-old Louis gathered on the balcony with King Charles and Queen Camilla to watch a Royal Air Force formation fly overhead. Kate, wearing one of her customary eye-catching hats, smiled and waved enthusiastically to the cheering crowd. A short time earlier the Princess of Wales and her children arrived in a state carriage f…