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.html

@@ -131,6 +131,15 @@
         <button id="btn-glb-load" class="wide">🧊 Carica un modello .glb…</button>
         <input type="file" id="glb-input" accept=".glb" hidden />
         <div class="hint-mini" id="glb-name">Il modello ruota a tempo di musica nella famiglia “Modello 3D”.</div>
+        <label>Sfondo Modello 3D</label>
+        <select id="glb-bg">
+          <option value="gradient">Gradiente</option>
+          <option value="ink">Fluido</option>
+          <option value="fire">Fluido Fuoco</option>
+          <option value="ring">Fluido Anello</option>
+          <option value="vortex">Fluido Vortici</option>
+          <option value="wave">Fluido Onda</option>
+        </select>
       </section>
 
       <section class="span2" id="effect-seq-section">