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/App.tsx

@@ -34,6 +34,7 @@ import AdvancedSearch from './pages/AdvancedSearch';
 import Playlists from './pages/Playlists';
 import PlaylistDetail from './pages/PlaylistDetail';
 import InternetRadio from './pages/InternetRadio';
+import FolderBrowser from './pages/FolderBrowser';
 import NowPlayingPage from './pages/NowPlaying';
 import FullscreenPlayer from './components/FullscreenPlayer';
 import ContextMenu from './components/ContextMenu';
@@ -352,6 +353,7 @@ function AppShell() {
             <Route path="/playlists" element={<Playlists />} />
             <Route path="/playlists/:id" element={<PlaylistDetail />} />
             <Route path="/radio" element={<InternetRadio />} />
+            <Route path="/folders" element={<FolderBrowser />} />
           </Routes>
         </div>
       </main>