feat: implement filtered book navigation and improve author sorting

- Sort authors by surname (last word in name) instead of first name
- Create FilteredLibraryItemsPage to display books filtered by author/genre/series
- Add navigation from authors/genres/series pages to filtered book lists
- Use AuthorFilter, GenreFilter, and SeriesFilter from shelfsdk
- Add libraryFiltered route with filter parameter support

Users can now:
- Browse authors sorted by surname
- Tap on an author to see all their books
- Tap on a genre to see all books in that genre
- Tap on a series to see all books in that series

lib/router/constants.dart

@@ -101,6 +101,12 @@ class Routes {
     parentRoute: libraryBrowser,
   );
 
+  static const libraryFiltered = _SimpleRoute(
+    pathName: 'filtered',
+    name: 'libraryFiltered',
+    parentRoute: libraryBrowser,
+  );
+
   // you page for the user
   static const you = _SimpleRoute(
     pathName: 'you',