Load FBX characters directly: in-app conversion to GLB

The model button now accepts .fbx too: a new model:convert IPC runs
Blender with scripts/fbx2modelglb.py (mesh + skin kept, animations
dropped, embedded diffuse recovered even when the FBX references
missing external textures) into userData/models3d, then the resulting
GLB loads as usual. anim/model conversion share one convertFbx helper.

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

src/control.html

@@ -163,8 +163,8 @@
       </section>
       <section class="span2">
         <label>Modello 3D (GLB)</label>
-        <button id="btn-glb-load" class="wide">🧊 Carica un modello .glb…</button>
-        <input type="file" id="glb-input" accept=".glb" hidden />
+        <button id="btn-glb-load" class="wide">🧊 Carica un modello (.glb / .fbx)…</button>
+        <input type="file" id="glb-input" accept=".glb,.fbx" hidden />
         <div class="hint-mini" id="glb-name">Il modello ruota e balla a tempo di musica nella famiglia “Modello 3D”;
           con la camera lo comandi in “Interattivo Avatar”.</div>
         <label>Sfondo</label>