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>

index.html

@@ -46,15 +46,6 @@
         <input id="backend-url" type="url" placeholder="http://192.168.1.x:3700">
         <div class="hint">Für Download-to-Server Funktion. Backend separat starten (novasonic-server.js)</div>
       </div>
-      <div class="field">
-        <label>Spotify Client ID <span style="color:var(--text3);font-weight:400;text-transform:none">(optional)</span></label>
-        <input id="spotify-client-id" type="text" placeholder="Für Spotify Importer">
-        <div class="hint">Kostenlos unter developer.spotify.com/dashboard</div>
-      </div>
-      <div class="field">
-        <label>Spotify Client Secret <span style="color:var(--text3);font-weight:400;text-transform:none">(optional)</span></label>
-        <input id="spotify-client-secret" type="password" placeholder="••••••••••••••••">
-      </div>
     </div>
     <button class="btn-primary" id="btn-connect" onclick="doConnect()">Verbinden</button>
     <div class="setup-error" id="setup-error"></div>
@@ -352,7 +343,7 @@
           <div>1. Spotify Link für Track, Album oder Playlist einfügen</div>
           <div>2. Metadaten werden direkt von Spotify geladen</div>
           <div>3. Songs via yt-dlp herunterladen → direkt in Navidrome Library</div>
-          <div style="margin-top:10px;color:var(--text3);font-size:12px">Spotify Client ID + Secret in den Einstellungen → Discover APIs eintragen.<br>Kostenlos unter <strong style="color:var(--text2)">developer.spotify.com/dashboard</strong></div>
+          <div style="margin-top:10px;color:var(--text3);font-size:12px">Kein Spotify-Account erforderlich — funktioniert direkt über das Backend.</div>
         </div>
         <div id="spotify-preview" style="display:none;margin-top:20px">
           <div id="spotify-preview-inner"></div>
@@ -518,15 +509,6 @@
           <input id="s-backend" type="url" placeholder="http://192.168.x.x:3700">
           <div class="hint">Für Download-to-Server via yt-dlp</div>
         </div>
-        <div class="field">
-          <label>Spotify Client ID</label>
-          <input id="s-spotify-id" type="text" placeholder="Für Spotify Importer">
-          <div class="hint">Kostenlos unter developer.spotify.com/dashboard</div>
-        </div>
-        <div class="field">
-          <label>Spotify Client Secret</label>
-          <input id="s-spotify-secret" type="password" placeholder="(unverändert lassen)">
-        </div>
         <div class="settings-row" style="margin-top:8px;border-radius:9px">
           <div>
             <div style="font-size:13px;font-weight:500">Backend Status</div>