feat(genres): local index genre browse with Subsonic fallback (#937)

* feat(genres): genre detail browse via local index with aligned counts

Move genre detail albums/play/shuffle onto the local library index with
Albums-style in-page scroll, session restore, and genre-scoped stash. Unify
genre album totals between the cloud and detail pages via
library_get_genre_album_counts, and fix grouped browse totals to count
distinct albums rather than matching tracks.

* perf(genres): local genre browse with scoped counts cache

Add dedicated Rust genre album pagination and indexes, slice-mode grid
loading, library-filter-aware counts, and a long-lived in-memory catalog
cache invalidated on sync so genre pages avoid repeated full-library SQL.

* fix(genres): restore scroll after album back; play hold-to-shuffle

Pin restore display count in refs so clearing the return stash no longer
reloads the genre grid mid-restore. Load the first SQL page only (60 rows),
use long-press on Play for shuffle, and add genre play tooltips.

* fix(genres): fall back to Subsonic byGenre when local index unavailable

Genre detail album grid now matches All Albums: try library_list_albums_by_genre
first, then getAlbumsByGenre when the index is off, not ready, or errors.

* docs: add CHANGELOG and credits for PR #937

src/api/library.ts

@@ -706,6 +706,12 @@ export type CatalogYearBounds = {
   maxYear: number | null;
 };
 
+export type GenreAlbumCountRow = {
+  value: string;
+  albumCount: number;
+  songCount: number;
+};
+
 export function libraryGetCatalogYearBounds(args: { serverId: string }): Promise<CatalogYearBounds> {
   const indexKey = serverIndexKeyForId(args.serverId);
   return invoke<CatalogYearBounds>('library_get_catalog_year_bounds', {
@@ -713,6 +719,58 @@ export function libraryGetCatalogYearBounds(args: { serverId: string }): Promise
   });
 }
 
+export function libraryGetGenreAlbumCounts(args: {
+  serverId: string;
+  libraryScope?: string;
+}): Promise<GenreAlbumCountRow[]> {
+  const indexKey = serverIndexKeyForId(args.serverId);
+  return invoke<GenreAlbumCountRow[]>('library_get_genre_album_counts', {
+    serverId: indexKey,
+    libraryScope: args.libraryScope,
+  });
+}
+
+export type LibraryGenreAlbumsRequest = {
+  serverId: string;
+  genre: string;
+  libraryScope?: string | null;
+  sort?: LibrarySortClause[];
+  limit?: number;
+  offset?: number;
+  includeTotal?: boolean;
+};
+
+export type LibraryGenreAlbumsResponse = {
+  albums: LibraryAlbumDto[];
+  hasMore: boolean;
+  total?: number | null;
+  source: 'local';
+};
+
+/** Paginated albums for one genre from the local track index. */
+export function libraryListAlbumsByGenre(
+  request: LibraryGenreAlbumsRequest,
+): Promise<LibraryGenreAlbumsResponse> {
+  const indexKey = serverIndexKeyForId(request.serverId);
+  return invoke<LibraryGenreAlbumsResponse>('library_list_albums_by_genre', {
+    request: {
+      serverId: indexKey,
+      genre: request.genre,
+      libraryScope: request.libraryScope ?? undefined,
+      sort: request.sort ?? [],
+      limit: request.limit ?? 50,
+      offset: request.offset ?? 0,
+      includeTotal: request.includeTotal ?? false,
+    },
+  }).then(response => ({
+    ...response,
+    albums: response.albums.map(album => ({
+      ...album,
+      serverId: mapServerIdFromIndexKey(album.serverId, request.serverId),
+    })),
+  }));
+}
+
 export type PlaySessionRecentDay = {
   date: string;
   totalListenedSec: number;