(#865) Lower the intersectionObserver threshold to 50%

Progress tracking doesn't trigger in infinite scroll when pages are too tall AND when you are using the mousewheel or similar. This happens because the IntersectionObserver threshold is set to 0.8, and is thus never triggered if 80% of a page is never visible at one time.

public/js/reader.js

@@ -268,7 +268,7 @@ Reader.initInfiniteScrollView = function () {
                 Reader.updateProgress();
             }
         }
-    }, { threshold: 0.8 });
+    }, { threshold: 0.5 });
 
     Reader.pages.slice(1).forEach((source) => {
         const img = new Image();