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/en.ts

@@ -22,6 +22,7 @@ export const enTranslation = {
     playlists: 'Playlists',
     mostPlayed: 'Most Played',
     radio: 'Internet Radio',
+    folderBrowser: 'Folder Browser',
     libraryScope: 'Library scope',
     allLibraries: 'All libraries',
   },
@@ -881,5 +882,9 @@ export const enTranslation = {
     favorite: 'Add to favorites',
     unfavorite: 'Remove from favorites',
     noFavorites: 'No favorite stations.',
-  }
+  },
+  folderBrowser: {
+    empty: 'Empty folder',
+    error: 'Failed to load',
+  },
 };