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/store/sidebarStore.ts

@@ -20,6 +20,7 @@ export const DEFAULT_SIDEBAR_ITEMS: SidebarItemConfig[] = [
   { id: 'playlists',     visible: true },
   { id: 'mostPlayed',    visible: true },
   { id: 'radio',         visible: true },
+  { id: 'folderBrowser', visible: false },
   { id: 'statistics',    visible: true },
   { id: 'help',          visible: true },
 ];