Only observe "ReaderChapterViewer" resize events for scroll position preservation

It's not necessary to observe all elements, since "ReaderChapterViewer" is the parent and thus increases/decreases in size in case its children change.

src/modules/reader/hooks/useReaderPreserveScrollPosition.ts

@@ -117,7 +117,6 @@ export const useReaderPreserveScrollPosition = (
         });
         mutationObserver.observe(element, {
             childList: true,
-            subtree: true,
         });
 
         return () => {