feat(folder-browser): Miller columns directory navigation

New page /folders with macOS Finder-style column layout:
- getMusicDirectory / getMusicIndexes / getMusicFolders Subsonic API methods
- Root level uses getIndexes.view (music folder IDs are not getMusicDirectory IDs)
- Columns fill available width with flex: 1, min-width: 200px
- Auto-scroll to rightmost column on expand
- Cancelled-flag prevents stale state on fast navigation
- Clicking a track plays it in the context of the current column's tracks
- FolderOpen/Folder/Music icons, accent-colored selection, ellipsis truncation
- Hidden in sidebar by default (user can enable in Settings)
- i18n: EN DE FR NL NB ZH RU

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/locales/de.ts

@@ -21,6 +21,7 @@ export const deTranslation = {
     playlists: 'Playlists',
     mostPlayed: 'Meistgehört',
     radio: 'Internetradio',
+    folderBrowser: 'Ordner-Browser',
     libraryScope: 'Bibliotheksumfang',
     allLibraries: 'Alle Bibliotheken',
   },
@@ -880,5 +881,9 @@ export const deTranslation = {
     favorite: 'Zu Favoriten hinzufügen',
     unfavorite: 'Aus Favoriten entfernen',
     noFavorites: 'Keine Lieblingssender.',
-  }
+  },
+  folderBrowser: {
+    empty: 'Leerer Ordner',
+    error: 'Laden fehlgeschlagen',
+  },
 };