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',

lib/router/router.dart

@@ -4,6 +4,7 @@ import 'package:vaani/features/downloads/view/downloads_page.dart';
 import 'package:vaani/features/explore/view/explore_page.dart';
 import 'package:vaani/features/explore/view/search_result_page.dart';
 import 'package:vaani/features/item_viewer/view/library_item_page.dart';
+import 'package:vaani/features/library_browser/view/filtered_library_items_page.dart';
 import 'package:vaani/features/library_browser/view/library_authors_page.dart';
 import 'package:vaani/features/library_browser/view/library_browser_page.dart';
 import 'package:vaani/features/library_browser/view/library_genres_page.dart';
@@ -140,6 +141,31 @@ class MyAppRouter {
                         name: Routes.librarySeries.name,
                         pageBuilder: defaultPageBuilder(const LibrarySeriesPage()),
                       ),
+                      GoRoute(
+                        path: Routes.libraryFiltered.pathName,
+                        name: Routes.libraryFiltered.name,
+                        pageBuilder: (context, state) {
+                          final extra = state.extra as Map<String, dynamic>?;
+                          final filter = extra?['filter'] as Filter?;
+                          final title = extra?['title'] as String? ?? 'Filtered Items';
+                          if (filter == null) {
+                            return MaterialPage(
+                              child: Scaffold(
+                                appBar: AppBar(title: const Text('Error')),
+                                body: const Center(
+                                  child: Text('Invalid filter'),
+                                ),
+                              ),
+                            );
+                          }
+                          return MaterialPage(
+                            child: FilteredLibraryItemsPage(
+                              filter: filter,
+                              title: title,
+                            ),
+                          );
+                        },
+                      ),
                     ],
                   ),
                 ],