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/main.js

@@ -14,7 +14,7 @@ import {
   navigate, _navGoBack, _navLoad, loadHomeData, onLocalSearch, onMobileSearch,
   onGlobalSearch, s_goArtist, s_goAlbum, playerBarGoArtist, playerBarGoAlbum,
   searchSimilarArtist, searchAndPreview, loadItunesAlbum, expandArtistBio,
-  loadRandom,
+  loadRandom, loadSpotifyImporter, resolveSpotifyLink, clearSpotifyPreview, spotifyDlAll,
 } from './views.js';
 import {
   openNowPlaying, closeNowPlaying, switchNpTab, npGoArtist, npGoAlbum,
@@ -22,23 +22,25 @@ import {
   checkBackend, openSidebar, closeSidebar, switchNavMode, showCtxMenu,
   openPlModal, closePlModal, addSongsToPlaylist, createAndAddPlaylist,
   createPlaylistPrompt, showSongInfo, closeInfoModal, openDlModal, closeDlModal,
-  selectDlType, executeDownload, openEditPlModal, closeEditPlModal,
+  selectDlType, executeDownload, executeSpotifyDownload, openEditPlModal, closeEditPlModal,
   saveEditPlaylist, editPlUploadCover, editPlCoverSelected, editPlRemoveCover,
   toggleMobileSearch, closeMobileSearch, switchSetupTab,
 } from './ui.js';
 
 // ─── CONNECT ───
 async function doConnect() {
-  const url     = document.getElementById('srv-url').value.trim().replace(/\/$/,'');
-  const user    = document.getElementById('srv-user').value.trim();
-  const pass    = document.getElementById('srv-pass').value;
-  const lfKey   = document.getElementById('lastfm-key').value.trim();
-  const backUrl = document.getElementById('backend-url').value.trim().replace(/\/$/,'');
+  const url      = document.getElementById('srv-url').value.trim().replace(/\/$/,'');
+  const user     = document.getElementById('srv-user').value.trim();
+  const pass     = document.getElementById('srv-pass').value;
+  const lfKey    = document.getElementById('lastfm-key').value.trim();
+  const backUrl  = document.getElementById('backend-url').value.trim().replace(/\/$/,'');
+  const spId     = document.getElementById('spotify-client-id').value.trim();
+  const spSecret = document.getElementById('spotify-client-secret').value.trim();
   if(!url || !user || !pass) { _showSetupError('Bitte Server-URL, Benutzernamen und Passwort ausfüllen.'); return; }
   const btn = document.getElementById('btn-connect');
   btn.disabled=true; btn.textContent='Verbinde…';
   document.getElementById('setup-error').style.display='none';
-  S.cfg = {url, user, pass, lastfmKey:lfKey, backendUrl:backUrl};
+  S.cfg = {url, user, pass, lastfmKey:lfKey, backendUrl:backUrl, spotifyClientId:spId, spotifyClientSecret:spSecret};
   try {
     await api('ping');
     localStorage.setItem('ns_cfg', JSON.stringify(S.cfg));
@@ -55,7 +57,7 @@ function disconnect() {
   localStorage.removeItem('ns_cfg');
   localStorage.removeItem('ns_last_played');
   sessionStorage.removeItem('ns_nav');
-  S.cfg = {url:'', user:'', pass:'', lastfmKey:'', backendUrl:''};
+  S.cfg = {url:'', user:'', pass:'', lastfmKey:'', backendUrl:'', spotifyClientId:'', spotifyClientSecret:''};
   S.queue=[]; S.qIdx=-1; S._navStack=[]; S._navCursor=-1;
   S.audio.pause(); S.audio.src='';
   document.getElementById('app').style.display='none';
@@ -183,7 +185,8 @@ Object.assign(window, {
   openSettings, closeSettings, saveSettings, checkBackend,
   openSidebar, closeSidebar, switchNavMode, showCtxMenu,
   // Views
-  loadRandom,
+  loadRandom, loadSpotifyImporter, resolveSpotifyLink, clearSpotifyPreview, spotifyDlAll,
+  executeSpotifyDownload,
   openPlModal, closePlModal, addSongsToPlaylist, createAndAddPlaylist, createPlaylistPrompt,
   showSongInfo, closeInfoModal,
   openDlModal, closeDlModal, selectDlType, executeDownload,