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/store/albumBrowseSessionStore.test.ts

@@ -5,8 +5,14 @@ import {
   DEFAULT_ALBUM_BROWSE_SORT,
   albumBrowseSortForServer,
   albumBrowseSurfaceForPath,
+  clearGenreDetailReturnStash,
   isAlbumDetailPath,
+  isGenreDetailPath,
+  genreDetailGenreFromPath,
   peekAlbumBrowseScrollRestore,
+  peekGenreDetailReturnStash,
+  peekGenreDetailScrollRestore,
+  stashGenreDetailReturnFilters,
   useAlbumBrowseSessionStore,
 } from './albumBrowseSessionStore';
 
@@ -97,6 +103,22 @@ describe('albumBrowseSessionStore', () => {
     const { sortByServer } = useAlbumBrowseSessionStore.getState();
     expect(albumBrowseSortForServer(sortByServer, 'unknown')).toBe(DEFAULT_ALBUM_BROWSE_SORT);
   });
+
+  it('stashes genre detail leave snapshot separately from album grid surfaces', () => {
+    stashGenreDetailReturnFilters('srv-a', 'Rock', {
+      ...DEFAULT_ALBUM_BROWSE_RETURN_FILTERS,
+      selectedGenres: ['Rock'],
+      scrollTop: 640,
+      displayCount: 90,
+    });
+    expect(peekGenreDetailReturnStash('srv-a', 'Rock')?.scrollTop).toBe(640);
+    expect(peekGenreDetailScrollRestore('srv-a', 'Rock')).toEqual({
+      scrollTop: 640,
+      displayCount: 90,
+    });
+    clearGenreDetailReturnStash('srv-a', 'Rock');
+    expect(peekGenreDetailReturnStash('srv-a', 'Rock')).toBeNull();
+  });
 });
 
 describe('isAlbumDetailPath', () => {
@@ -109,6 +131,16 @@ describe('isAlbumDetailPath', () => {
   });
 });
 
+describe('isGenreDetailPath', () => {
+  it('matches single genre detail routes only', () => {
+    expect(isGenreDetailPath('/genres/Rock')).toBe(true);
+    expect(isGenreDetailPath('/genres/Rock%20%26%20Roll')).toBe(true);
+    expect(isGenreDetailPath('/genres')).toBe(false);
+    expect(isGenreDetailPath('/genres/Rock/albums')).toBe(false);
+    expect(genreDetailGenreFromPath('/genres/Rock%20%26%20Roll')).toBe('Rock & Roll');
+  });
+});
+
 describe('albumBrowseSurfaceForPath', () => {
   it('maps album grid browse routes', () => {
     expect(albumBrowseSurfaceForPath('/albums')).toBe('albums');