feat: replace orb avatar with a livekit-style LED wall driven by agent audio

- Swap the CSS orb for a 21x21 LED dot matrix on a glass panel, matching
  the livekit.com LED-array hero visual.
- Pulse driven by the agent's rendered audio: an expanding radial bloom
  brightens and widens with remote-audio level, dimmer when listening on
  the mic, faint static matrix when idle, and a subtle warm breathing
  state while thinking.
- State-tinted palettes: cool azure when speaking, blue when listening,
  amber when thinking, dim slate when idle.
- Drop the unused orb CSS-variable pulse plumbing and the presence-breathe
  / orbit-turn keyframes; the wall renders per-frame from the analyser.

web/index.html

@@ -58,12 +58,7 @@
                 </div>
 
                 <div class="orb-stage">
-                    <div class="orb" id="orb" data-state="idle" aria-hidden="true">
-                        <div class="orb-orbit orbit-one"></div>
-                        <div class="orb-orbit orbit-two"></div>
-                        <div class="orb-core"><span class="orb-shimmer"></span></div>
-                        <div class="orb-ring"></div>
-                    </div>
+                    <div class="led-wall" id="ledWall" data-state="idle" aria-hidden="true"></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>