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/store/sidebarStore.ts

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