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/views.js

@@ -917,9 +917,7 @@ export function clearSpotifyPreview() {
 export async function resolveSpotifyLink() {
   const url = document.getElementById('spotify-link-input')?.value.trim();
   if (!url) return;
-  if (!S.cfg.backendUrl)           { toast('⚠ Backend URL in Einstellungen konfigurieren'); return; }
-  if (!S.cfg.spotifyClientId)      { toast('⚠ Spotify Client ID in Einstellungen eintragen'); return; }
-  if (!S.cfg.spotifyClientSecret)  { toast('⚠ Spotify Client Secret in Einstellungen eintragen'); return; }
+  if (!S.cfg.backendUrl) { toast('⚠ Backend URL in Einstellungen konfigurieren'); return; }
 
   const hint    = document.getElementById('spotify-importer-hint');
   const preview = document.getElementById('spotify-preview');
@@ -935,7 +933,7 @@ export async function resolveSpotifyLink() {
     const res = await fetch(`${S.cfg.backendUrl}/api/spotify/resolve`, {
       method: 'POST',
       headers: { 'Content-Type': 'application/json' },
-      body: JSON.stringify({ url, clientId: S.cfg.spotifyClientId, clientSecret: S.cfg.spotifyClientSecret }),
+      body: JSON.stringify({ url }),
     });
     if (!res.ok) {
       const err = await res.json().catch(() => ({}));