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/store/authStore.ts

@@ -57,6 +57,16 @@ interface AuthState {
   /** Parent directory; actual cache is `<dir>/psysonic-hot-cache/`. Empty = app data. */
   hotCacheDownloadDir: string;
 
+  /** Subsonic music folders for the active server (not persisted; refetched on login / server change). */
+  musicFolders: Array<{ id: string; name: string }>;
+  /**
+   * Per server: `all` = no musicFolderId param; otherwise a single folder id.
+   * Only one library or all — no multi-folder merge.
+   */
+  musicLibraryFilterByServer: Record<string, 'all' | string>;
+  /** Bumps when `setMusicLibraryFilter` runs so pages refetch catalog data. */
+  musicLibraryFilterVersion: number;
+
   // Status
   isLoggedIn: boolean;
   isConnecting: boolean;
@@ -103,6 +113,8 @@ interface AuthState {
   setHotCacheMaxMb: (v: number) => void;
   setHotCacheDebounceSec: (v: number) => void;
   setHotCacheDownloadDir: (v: string) => void;
+  setMusicFolders: (folders: Array<{ id: string; name: string }>) => void;
+  setMusicLibraryFilter: (folderId: 'all' | string) => void;
   logout: () => void;
 
   // Derived
@@ -151,6 +163,9 @@ export const useAuthStore = create<AuthState>()(
       hotCacheMaxMb: 256,
       hotCacheDebounceSec: 30,
       hotCacheDownloadDir: '',
+      musicFolders: [],
+      musicLibraryFilterByServer: {},
+      musicLibraryFilterVersion: 0,
       isLoggedIn: false,
       isConnecting: false,
       connectionError: null,
@@ -180,7 +195,7 @@ export const useAuthStore = create<AuthState>()(
         });
       },
 
-      setActiveServer: (id) => set({ activeServerId: id }),
+      setActiveServer: (id) => set({ activeServerId: id, musicFolders: [] }),
 
       setLoggedIn: (v) => set({ isLoggedIn: v }),
       setConnecting: (v) => set({ isConnecting: v }),
@@ -233,7 +248,30 @@ export const useAuthStore = create<AuthState>()(
       setHotCacheDebounceSec: (v) => set({ hotCacheDebounceSec: v }),
       setHotCacheDownloadDir: (v) => set({ hotCacheDownloadDir: v }),
 
-      logout: () => set({ isLoggedIn: false }),
+      setMusicFolders: (folders) => {
+        const sid = get().activeServerId;
+        set(s => {
+          const f = sid ? s.musicLibraryFilterByServer[sid] : undefined;
+          const invalidFilter = f && f !== 'all' && !folders.some(x => x.id === f);
+          return {
+            musicFolders: folders,
+            ...(sid && invalidFilter
+              ? { musicLibraryFilterByServer: { ...s.musicLibraryFilterByServer, [sid]: 'all' } }
+              : {}),
+          };
+        });
+      },
+
+      setMusicLibraryFilter: (folderId) => {
+        const sid = get().activeServerId;
+        if (!sid) return;
+        set(s => ({
+          musicLibraryFilterByServer: { ...s.musicLibraryFilterByServer, [sid]: folderId },
+          musicLibraryFilterVersion: s.musicLibraryFilterVersion + 1,
+        }));
+      },
+
+      logout: () => set({ isLoggedIn: false, musicFolders: [] }),
 
       getBaseUrl: () => {
         const s = get();
@@ -250,6 +288,10 @@ export const useAuthStore = create<AuthState>()(
     {
       name: 'psysonic-auth',
       storage: createJSONStorage(() => localStorage),
+      partialize: state => {
+        const { musicFolders: _mf, musicLibraryFilterVersion: _fv, ...rest } = state;
+        return rest;
+      },
     }
   )
 );