fix: solid cover-color NP background via canvas extraction

Replace saturated blur filter with actual dominant-color extraction:
fetch cover as blob (avoids canvas CORS taint), draw on 24x24 canvas,
find most vivid pixel, darken to ~28% for a moody solid background.
Blurred bg layers hidden when mode is active. Transitions smoothly
between songs via CSS transition on background.
Renamed setting label to "Einfarbiger Player-Hintergrund".

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

index.html

@@ -664,10 +664,7 @@
         </h3>
         <div id="theme-picker" class="theme-picker"></div>
         <div class="edit-pl-toggle-row" style="margin-top:14px">
-          <div>
-            <span>Dynamische Hintergrundfarbe</span>
-            <div class="hint" style="margin-top:2px">Jetzt-Läuft-Hintergrund nach Cover-Hauptfarbe</div>
-          </div>
+          <span>Einfarbiger Player-Hintergrund</span>
           <div id="s-cover-color-bg" class="toggle" onclick="this.classList.toggle('on')"></div>
         </div>
       </div>