fix: prevent returning to previous book when exiting reader after auto-advance

Use router.replace instead of router.push when auto-advancing to next book,
so closing the reader navigates back to the series view instead of the previous book.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/components/reader/hooks/usePageNavigation.ts

@@ -99,7 +99,7 @@ export function usePageNavigation({
   const handleNextPage = useCallback(() => {
     if (currentPage === pages.length) {
       if (nextBook) {
-        router.push(`/books/${nextBook.id}`);
+        router.replace(`/books/${nextBook.id}`);
         return;
       }
       setShowEndMessage(true);