generate_3d: 3D models with fal.ai (image, text, or image steered by text)

- New tool: image-to-3D (project file or an image attached to the chat,
  optional back/left/right views) or text-to-3D; image + prompt refines the
  image with the prompt first (the 3D models take no text with an image).
  Styles textured / geometry / lowpoly, optional PBR, face count. Runs through
  fal.ai's queue API with progress, saves GLB + preview, books the cost
  (published prices, overridable per model). Request bodies for the Hunyuan3D,
  Tripo and Trellis families; defaults [images] model_3d / model_3d_text.
- Chat card: "Open in the 3D view"; media overview: kind "model" (also
  STL/3MF/OBJ written by agents) with previews.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

server/src/tools/mod.rs

@@ -177,6 +177,7 @@ pub fn builtin_tools() -> Vec<Arc<dyn Tool>> {
         Arc::new(craft::ImageCompare),
         Arc::new(crate::prompts::SkillTool),
         Arc::new(crate::images::GenerateImage),
+        Arc::new(crate::mesh3d::Generate3d),
         Arc::new(crate::reverse::BinaryInfo),
         Arc::new(crate::reverse::Decompile),
     ]