fix: wire bell emoji slot to HUD notification icon

The admin had a 'bell' emoji slot but the HUD used a hardcoded SVG.
Now uses emoji('bell', '🔔', 18) so uploaded PNGs actually appear.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

public/js/core/hud.js

@@ -46,10 +46,7 @@ function renderHud() {
     </div>
     <div class="hud-actions">
       <div class="hud-btn" id="hud-notif">
-        <svg width="18" height="18" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2">
-          <path d="M18 8A6 6 0 0 0 6 8c0 7-3 9-3 9h18s-3-2-3-9"></path>
-          <path d="M13.73 21a2 2 0 0 1-3.46 0"></path>
-        </svg>
+        ${emoji('bell', '🔔', 18)}
         <div class="hud-badge" id="hud-badge"></div>
       </div>
     </div>