fix: use Spotify public web-player token — no Premium/API account needed

Replace Client Credentials flow with Spotify's public web-player token endpoint
(open.spotify.com/get_access_token) which works without any account or Premium.
Remove Spotify credential fields from UI entirely.

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

src/ui.js

@@ -61,8 +61,6 @@ export function openSettings() {
   document.getElementById('s-pass').value    = '';
   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();
 }
@@ -74,9 +72,6 @@ export function saveSettings() {
   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.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');
 }