The lightbox showed a "Swipe to navigate" hint on mobile, but the touch
handlers only implemented pinch-to-zoom (2-finger). Single-finger swipe
fell through and the user could only navigate with the on-screen arrows.
Add a 1-finger swipe detector: track the initial touch position, and on
touchEnd compute deltaX/deltaY/duration. Trigger goToPrevious /
goToNext when the horizontal swipe exceeds 50px, dominates over
vertical motion (1.2x), and completes within 600ms. Suppressed while
zoomed in so the user can pan the image instead.
Paul Nothaft
4c8eba0cb4