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/output.js

@@ -320,6 +320,7 @@ djv.onControl(async (m) => {
       djv.report({ type: 'autoVj', on: autoVj });
       break;
     case 'svg': loadCustomTexture(m.dataUrl); break;
+    case 'modelBg': viz.modelBg = m.mode || 'gradient'; break;
     case 'glb':
       try {
         const bytes = await djv.readFile(m.path);