feat(subsonic): per-server music folder filter and sidebar picker (#125)

Adds library scoping per server: users can select one Navidrome music
folder or 'All' from a new sidebar dropdown. Choice is persisted per
server ID in localStorage. A musicLibraryFilterVersion counter triggers
refetches across browsing pages when the scope changes.

- getMusicFolders() + libraryFilterParams() in subsonic.ts
- New fields in authStore: musicFolders, musicLibraryFilterByServer,
  musicLibraryFilterVersion, setMusicFolders, setMusicLibraryFilter
- Sidebar dropdown via createPortal; collapses to icon in narrow mode
- API scoping applied to: getAlbumList2, getRandomSongs, getArtists,
  getStarred2, search3
- Full i18n coverage (en, de, fr, nl, zh, nb, ru)

Review fixes applied (co-authored):
- Sidebar: title={} → data-tooltip + data-tooltip-pos='right'
- authStore: setMusicFolders merged into single set() call
- Locales: removed dead libraryScopeHint key from all 7 files
- Genres.tsx: removed musicLibraryFilterVersion dep (getGenres unscoped)

Co-authored-by: cucadmuh <cucadmuh@users.noreply.github.com>
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/locales/de.ts

@@ -20,6 +20,8 @@ export const deTranslation = {
     genres: 'Genres',
     playlists: 'Playlists',
     radio: 'Internetradio',
+    libraryScope: 'Bibliotheksumfang',
+    allLibraries: 'Alle Bibliotheken',
   },
   home: {
     hero: 'Featured',