add mobile swipe navigation in fullscreen

templates/slideshow.html

@@ -203,6 +203,20 @@
             z_drag=null;
             if(document.fullscreenElement && z_scale>1) img.style.cursor='grab';
         });
+        let t_start_x = 0, t_start_y = 0;
+        img.addEventListener('touchstart', e => {
+            if (!document.fullscreenElement) return;
+            t_start_x = e.touches[0].clientX;
+            t_start_y = e.touches[0].clientY;
+        }, { passive: true });
+        img.addEventListener('touchend', e => {
+            if (!document.fullscreenElement) return;
+            const dx = e.changedTouches[0].clientX - t_start_x;
+            const dy = e.changedTouches[0].clientY - t_start_y;
+            if (Math.abs(dx) > Math.abs(dy) && Math.abs(dx) > 50) {
+                dx < 0 ? next() : prev();
+            }
+        }, { passive: true });
         document.addEventListener('fullscreenchange', ()=>{
             if(!document.fullscreenElement) reset_zoom();
         });