feat(device-sync): USB/SD card sync page (WIP)

Adds a new Device Sync page for transferring music from Navidrome to
USB drives or SD cards. Sidebar entry is hidden by default.

- Four new Tauri commands: sync_track_to_device, compute_sync_paths,
  list_device_dir_files, delete_device_file
- Filename template engine with cross-platform path sanitization
- 4-concurrent-worker download, live progress via device:sync:progress event
- Persistent source list (albums/playlists/artists) with checkbox deletion
- Expandable artist tree in browser panel (per-album selection)
- i18n: DE + EN complete; other locales have stub keys

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

src/App.tsx

@@ -36,6 +36,7 @@ import Playlists from './pages/Playlists';
 import PlaylistDetail from './pages/PlaylistDetail';
 import InternetRadio from './pages/InternetRadio';
 import FolderBrowser from './pages/FolderBrowser';
+import DeviceSync from './pages/DeviceSync';
 import NowPlayingPage from './pages/NowPlaying';
 import FullscreenPlayer from './components/FullscreenPlayer';
 import ContextMenu from './components/ContextMenu';
@@ -370,6 +371,7 @@ function AppShell() {
             <Route path="/playlists/:id" element={<PlaylistDetail />} />
             <Route path="/radio" element={<InternetRadio />} />
             <Route path="/folders" element={<FolderBrowser />} />
+            <Route path="/device-sync" element={<DeviceSync />} />
           </Routes>
         </div>
       </main>