feat: Spotify Importer — link → yt-dlp download + optional Navidrome playlist

- New 'Spotify Importer' page under Entdecken (sidebar + mobile nav)
- Backend: POST /api/spotify/resolve — resolves track/album/playlist URLs via Spotify API
- Backend: new 'playlist' download type accepting pre-resolved track list (no iTunes re-fetch)
- Frontend: paste Spotify link → preview with cover, type badge, track list
  - Track: single download or full album via iTunes fallback
  - Album: download all tracks with Spotify metadata
  - Playlist: download all tracks + optional 'Als Navidrome-Playlist speichern' checkbox
- Navidrome playlist creation: searches library by title/artist after download, creates playlist
- Spotify Client ID + Secret stored in app settings (setup screen + settings modal)
- iTunes metadata tags kept for all downloaded files (existing pipeline unchanged)

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

src/state.js

@@ -1,6 +1,6 @@
 // Shared mutable state — imported as { S } in every module
 export const S = {
-  cfg: { url: '', user: '', pass: '', lastfmKey: '', backendUrl: '' },
+  cfg: { url: '', user: '', pass: '', lastfmKey: '', backendUrl: '', spotifyClientId: '', spotifyClientSecret: '' },
 
   // Playback
   queue: [],