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/ui.js

@@ -59,8 +59,10 @@ export function openSettings() {
   document.getElementById('s-url').value     = S.cfg.url || '';
   document.getElementById('s-user').value    = S.cfg.user || '';
   document.getElementById('s-pass').value    = '';
-  document.getElementById('s-lastfm').value  = S.cfg.lastfmKey || '';
-  document.getElementById('s-backend').value = S.cfg.backendUrl || '';
+  document.getElementById('s-lastfm').value       = S.cfg.lastfmKey        || '';
+  document.getElementById('s-backend').value      = S.cfg.backendUrl       || '';
+  document.getElementById('s-spotify-id').value   = S.cfg.spotifyClientId  || '';
+  document.getElementById('s-spotify-secret').value = '';
   document.getElementById('settings-modal').classList.add('open');
   checkBackend();
 }
@@ -70,8 +72,11 @@ export function saveSettings() {
   S.cfg.url       = document.getElementById('s-url').value.trim().replace(/\/$/,'');
   S.cfg.user      = document.getElementById('s-user').value.trim();
   if(pass) S.cfg.pass = pass;
-  S.cfg.lastfmKey  = document.getElementById('s-lastfm').value.trim();
-  S.cfg.backendUrl = document.getElementById('s-backend').value.trim().replace(/\/$/,'');
+  S.cfg.lastfmKey        = document.getElementById('s-lastfm').value.trim();
+  S.cfg.backendUrl       = document.getElementById('s-backend').value.trim().replace(/\/$/,'');
+  S.cfg.spotifyClientId  = document.getElementById('s-spotify-id').value.trim();
+  const spSec = document.getElementById('s-spotify-secret').value;
+  if (spSec) S.cfg.spotifyClientSecret = spSec;
   localStorage.setItem('ns_cfg', JSON.stringify(S.cfg));
   closeSettings(); toast('✅ Gespeichert');
 }
@@ -278,6 +283,67 @@ export async function executeDownload() {
   }
 }
 
+// ─── SPOTIFY DOWNLOAD ───
+export async function executeSpotifyDownload({ type, title, artist, artworkUrl, tracks, createPlaylist, playlistName }) {
+  if (!S.cfg.backendUrl) { toast('⚠ Kein Backend konfiguriert'); return; }
+  const jobId = 'job_' + Date.now();
+  addJob(jobId, title, artist);
+  try {
+    const res = await fetch(`${S.cfg.backendUrl}/api/download`, {
+      method: 'POST',
+      headers: { 'Content-Type': 'application/json' },
+      body: JSON.stringify({ jobId, type, title, artist, artworkUrl, tracks }),
+    });
+    if (!res.ok) throw new Error(`HTTP ${res.status}`);
+    pollJob(jobId);
+    if (createPlaylist && playlistName && tracks?.length) {
+      _pollAndCreatePlaylist(jobId, tracks, playlistName);
+    }
+  } catch(e) {
+    updateJob(jobId, 'error', 'Backend-Verbindung fehlgeschlagen: ' + e.message);
+    toast('❌ Backend nicht erreichbar: ' + e.message);
+  }
+}
+
+async function _pollAndCreatePlaylist(jobId, tracks, playlistName) {
+  while (true) {
+    await new Promise(r => setTimeout(r, 2500));
+    const job = S.jobs[jobId];
+    if (!job || job.status === 'done' || job.status === 'error') break;
+  }
+  if (S.jobs[jobId]?.status !== 'done') return;
+  await _createNdPlaylist(tracks, playlistName);
+}
+
+async function _createNdPlaylist(tracks, name) {
+  try {
+    toast('🎵 Erstelle Navidrome-Playlist…');
+    const songIds = [];
+    const norm = s => s?.toLowerCase().trim() || '';
+    for (const t of tracks) {
+      if (!t.trackName) continue;
+      try {
+        const r = await api('search3', `&query=${encodeURIComponent(t.trackName)}&songCount=5&albumCount=0&artistCount=0`);
+        const songs = r.searchResult3?.song || [];
+        const match = songs.find(s =>
+          norm(s.title) === norm(t.trackName) &&
+          norm(s.artist).split(' ')[0] === norm(t.artistName).split(' ')[0]
+        ) || songs.find(s => norm(s.title) === norm(t.trackName));
+        if (match) songIds.push(match.id);
+      } catch {}
+    }
+    if (!songIds.length) {
+      toast('⚠ Keine Tracks in Navidrome gefunden — zuerst Library scannen, dann Playlist manuell anlegen');
+      return;
+    }
+    const songParams = songIds.map(id => `&songId=${encodeURIComponent(id)}`).join('');
+    await api('createPlaylist', `&name=${encodeURIComponent(name)}${songParams}`);
+    toast(`✅ Playlist "${name}" mit ${songIds.length}/${tracks.length} Tracks erstellt`);
+  } catch(e) {
+    toast('Playlist-Erstellung fehlgeschlagen: ' + e.message);
+  }
+}
+
 // ─── JOBS ───
 function addJob(id, title, artist) {
   S.jobs[id] = {title, artist, progress:0, status:'pending'};