[fix](trx-frontend-http): restore select_rig call and simplify play button icon

Rig switch needs the server call so SSE/audio follow the selected rig. Play button now uses a fixed triangle icon sized to match header controls.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: Stan Grams <sjg@haxx.space>

src/trx-client/trx-frontend/trx-frontend-http/assets/web/index.html

@@ -59,7 +59,7 @@
       </div>
       <div class="top-bar-actions">
         <button id="header-audio-toggle" class="header-bar-btn header-audio-btn" aria-label="Toggle audio playback" title="Toggle audio playback">
-          <svg viewBox="0 0 16 16" width="14" height="14" fill="currentColor" aria-hidden="true"><path id="header-audio-icon-path" d="M5 3.5v9l7-4.5z"/></svg>
+          <svg viewBox="0 0 16 16" fill="currentColor" aria-hidden="true"><path d="M5 3v10l8-5z"/></svg>
         </button>
         <div class="header-rig-switch">
           <select id="header-rig-switch-select" aria-label="Select active rig"></select>