fix: clear background on back press (before transition), native-like

plugin.js

@@ -376,6 +376,18 @@
                             }
                         },
                         back: function () {
+                            // Clear our CSS background immediately (before the
+                            // back transition), matching native Lampa behaviour.
+                            try {
+                                var bgel = document.querySelector('.background');
+                                if (bgel) {
+                                    bgel.style.backgroundImage = '';
+                                    bgel.style.filter = '';
+                                    bgel.style.transform = '';
+                                    bgel.style.opacity = '';
+                                }
+                                if (Lampa.Background && Lampa.Background.immediately) Lampa.Background.immediately('');
+                            } catch (e) {}
                             Lampa.Activity.backward();
                         },
                         right: function () { Navigator.move('right'); },