feat: show album art in the avatar square while music is loaded

The LED wall square switches to the track's cover art whenever a song is
loaded outside a conversation, and returns to the pulsing LED wall while
speaking to the agent (music and conversation are mutually exclusive, so
starting a talk hides the art and ending it restores it).

web/index.html

@@ -58,7 +58,9 @@
                 </div>
 
                 <div class="orb-stage">
-                    <div class="led-wall" id="ledWall" data-state="idle" aria-hidden="true"></div>
+                    <div class="led-wall" id="ledWall" data-state="idle" aria-hidden="true">
+                        <img id="ledArt" class="led-art" alt="">
+                    </div>
                     <div class="orb-labels" aria-live="polite">
                         <span class="state-label" id="stateLabel">Ready when you are</span>
                         <span class="thinking-chip" id="thinkingChip" hidden>Thinking&hellip;</span>