fix(chat): add missing chatTtsToggle button to prevent null reference error

The chat.js init() template did not include #chatTtsToggle, causing
a runtime TypeError when hasTTS was true. Added the toggle button
inside .chat-header-actions, matching chat-public.js structure.
Regenerated chat.min.js.

pos/static/js/chat.js

@@ -32,8 +32,11 @@
     panel.innerHTML = `
       <div class="chat-header">
         <h3>Asistente IA — Buscar partes</h3>
+        <div class="chat-header-actions">
+          ${hasTTS ? '<button class="chat-tts-toggle" id="chatTtsToggle" aria-label="Activar lectura de respuestas" title="Activar lectura de respuestas">&#128266;</button>' : ''}
           <button class="chat-header-close" id="chatClose" aria-label="Cerrar">&times;</button>
         </div>
+      </div>
       <div class="chat-messages" id="chatMessages">
         <div class="chat-msg ai">Hola, soy el asistente de Nexus. Dime que refaccion necesitas y te ayudo a encontrarla.</div>
         <div class="chat-typing" id="chatTyping">