feat: implement library view with Authors, Genres, and Series browsing

This commit implements a comprehensive library browsing feature:

- Add LibraryBrowserProvider with providers for authors, genres, and series data
- Create LibraryAuthorsPage with grid view of authors including images and book counts
- Create LibraryGenresPage with list view of all genres
- Create LibrarySeriesPage with list view of series and book counts
- Update LibraryBrowserPage navigation to route to the new views
- Add routes for /browser/authors, /browser/genres, and /browser/series
- Replace "Not Implemented" toasts with functional navigation

The implementation uses the Audiobookshelf API via shelfsdk to fetch:
- Authors list with metadata (getAuthors)
- Genres from library filter data (getFilterData)
- Series with pagination support (getSeries)

All views follow Material Design 3 patterns and include proper loading/error states.

lib/api/library_browser_provider.dart

@@ -0,0 +1,79 @@
+import 'package:hooks_riverpod/hooks_riverpod.dart' show Ref;
+import 'package:logging/logging.dart' show Logger;
+import 'package:riverpod_annotation/riverpod_annotation.dart';
+import 'package:shelfsdk/audiobookshelf_api.dart'
+    show Author, GetLibrarysSeriesResponse, LibraryFilterData;
+import 'package:vaani/api/api_provider.dart' show authenticatedApiProvider;
+import 'package:vaani/api/library_provider.dart' show currentLibraryProvider;
+
+part 'library_browser_provider.g.dart';
+
+final _logger = Logger('LibraryBrowserProvider');
+
+/// Provider for fetching all authors in the current library
+@riverpod
+Future<List<Author>> libraryAuthors(Ref ref) async {
+  final api = ref.watch(authenticatedApiProvider);
+  final currentLibrary = await ref.watch(currentLibraryProvider.future);
+
+  if (currentLibrary == null) {
+    _logger.warning('No current library found');
+    return [];
+  }
+
+  final authors = await api.libraries.getAuthors(libraryId: currentLibrary.id);
+
+  if (authors == null) {
+    _logger.warning('Failed to fetch authors for library ${currentLibrary.id}');
+    return [];
+  }
+
+  _logger.fine('Fetched ${authors.length} authors');
+  return authors;
+}
+
+/// Provider for fetching all genres in the current library
+@riverpod
+Future<List<String>> libraryGenres(Ref ref) async {
+  final api = ref.watch(authenticatedApiProvider);
+  final currentLibrary = await ref.watch(currentLibraryProvider.future);
+
+  if (currentLibrary == null) {
+    _logger.warning('No current library found');
+    return [];
+  }
+
+  final filterData = await api.libraries.getFilterData(libraryId: currentLibrary.id);
+
+  if (filterData == null) {
+    _logger.warning('Failed to fetch filter data for library ${currentLibrary.id}');
+    return [];
+  }
+
+  _logger.fine('Fetched ${filterData.genres.length} genres');
+  return filterData.genres;
+}
+
+/// Provider for fetching all series in the current library
+@riverpod
+Future<GetLibrarysSeriesResponse?> librarySeries(Ref ref, {int page = 0, int limit = 50}) async {
+  final api = ref.watch(authenticatedApiProvider);
+  final currentLibrary = await ref.watch(currentLibraryProvider.future);
+
+  if (currentLibrary == null) {
+    _logger.warning('No current library found');
+    return null;
+  }
+
+  final series = await api.libraries.getSeries(
+    libraryId: currentLibrary.id,
+  );
+
+  if (series == null) {
+    _logger.warning('Failed to fetch series for library ${currentLibrary.id}');
+    return null;
+  }
+
+  _logger.fine('Fetched ${series.results.length} series (${series.total} total)');
+  return series;
+}