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/zh.ts

@@ -21,6 +21,7 @@ export const zhTranslation = {
     playlists: '播放列表',
     mostPlayed: '最常播放',
     radio: '网络电台',
+    folderBrowser: '文件夹浏览器',
     libraryScope: '资料库范围',
     allLibraries: '所有资料库',
   },
@@ -871,5 +872,9 @@ export const zhTranslation = {
     favorite: '添加到收藏',
     unfavorite: '从收藏移除',
     noFavorites: '没有收藏的电台。',
-  }
+  },
+  folderBrowser: {
+    empty: '空文件夹',
+    error: '加载失败',
+  },
 };