Novelist Nicholas Sparks’ formula of life-altering love — invariably faced with some grand, operatic impediment — receives the youth-edition treatment in “Dear John,” an almost painfully earnest romance that throws the Sept. 11 terror attacks between its Romeo and Juliet. Mostly, the movie provides ample opportunity to admire Channing Tatum’s broad shoulders and Amanda Seyfried’s incandescent smile, but the narrative device that keeps them connected while geographically apart doesn’t work especially well onscreen. The result amounts to a handsome Lifetime movie that could do some modest business before taking its place on the lower shelves of the Sparks bibliography.