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/runner.rs

@@ -215,7 +215,7 @@ pub fn available_tools(app: &App, agent: &Agent, owner: Option<&crate::store::Us
     let cfg = app.config();
     let mut all = tools::builtin_tools();
     if !cfg.images.enabled {
-        all.retain(|t| t.name() != "generate_image");
+        all.retain(|t| t.name() != "generate_image" && t.name() != "generate_3d");
     }
     let (direct, deferred) = app.mcp.tools(&cfg, owner);
     all.extend(direct);