Fluid backdrop behind the 3D model

New 'Sfondo Modello 3D' selector (Media tab): gradient or any of the
five fluid modes. The fluid sim paints the canvas first, then the model
renders on top clearing only the depth buffer (overlay mode skips its
own backdrop pass). Palette and speed are shared with the preset;
persisted and re-sent when the output restarts.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/control.js

@@ -146,13 +146,23 @@ $('#svg-input').addEventListener('change', (e) => {
 // --- 3D model (GLB) for the 'Modello 3D' family -----------------------------
 // The path persists and is re-sent when the output window (re)starts.
 let glbPath = localStorage.getItem('glb3d') || '';
-function glbSend() { if (glbPath) send({ type: 'glb', path: glbPath }); }
+let glbBg = localStorage.getItem('glbbg') || 'gradient';
+function glbSend() {
+  if (glbPath) send({ type: 'glb', path: glbPath });
+  send({ type: 'modelBg', mode: glbBg });
+}
 function glbLabel(name) {
   $('#glb-name').textContent = name
     ? '🧊 ' + name + ' — attiva la famiglia “Modello 3D” per vederlo.'
     : 'Il modello ruota a tempo di musica nella famiglia “Modello 3D”.';
 }
 if (glbPath) glbLabel(glbPath.split('/').pop());
+$('#glb-bg').value = glbBg;
+$('#glb-bg').addEventListener('change', (e) => {
+  glbBg = e.target.value;
+  localStorage.setItem('glbbg', glbBg);
+  send({ type: 'modelBg', mode: glbBg });
+});
 $('#btn-glb-load').addEventListener('click', () => $('#glb-input').click());
 $('#glb-input').addEventListener('change', (e) => {
   const f = e.target.files[0];