Add on-air toggle for phone call routing

When off air, callers hear a message and get disconnected. When on
air, calls route normally. Toggle button added to frontend header
with pulsing red ON AIR indicator.

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

frontend/index.html

@@ -11,6 +11,7 @@
         <header>
             <h1>Luke at The Roost</h1>
             <div class="header-buttons">
+                <button id="on-air-btn" class="on-air-btn off">OFF AIR</button>
                 <button id="new-session-btn" class="new-session-btn">New Session</button>
                 <button id="settings-btn">Settings</button>
             </div>