feat(io): FBXEXPORT writes the drawing's 3D meshes as binary FBX

FBXEXPORT (also EXPORTFBX) saves every 3D mesh of the drawing (solids,
surfaces and meshes, at full display resolution) as a binary FBX 7.4
file for visualisation tools: one model per object with its geometry,
face normals and a material in its display colour. Axes stay Z-up and
the unit scale follows the drawing's insertion units.

Refs #1502

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

src/app/update/mod.rs

@@ -1994,6 +1994,62 @@ impl OpenCADStudio {
                 }
             }
 
+            // ── FBX export ────────────────────────────────────────────────
+            Message::FbxExport => {
+                let i = self.active_tab;
+                if self.tabs[i].scene.meshes.is_empty() {
+                    self.command_line.push_error(
+                        crate::t!("FBXEXPORT: no 3D mesh data in this drawing.").as_ref(),
+                    );
+                    return Task::none();
+                }
+                #[cfg(not(target_arch = "wasm32"))]
+                {
+                    Task::perform(
+                        async {
+                            crate::sys::file_dialog()
+                                .set_title(crate::t!("Export FBX").as_ref())
+                                .set_file_name("export.fbx")
+                                .add_filter(crate::t!("FBX Files").as_ref(), &["fbx"])
+                                .add_filter(crate::t!("All Files").as_ref(), &["*"])
+                                .save_file()
+                                .await
+                                .map(|h| crate::sys::handle_path(&h))
+                        },
+                        Message::FbxExportPath,
+                    )
+                }
+                #[cfg(target_arch = "wasm32")]
+                {
+                    let stem = self.tabs[i]
+                        .current_path
+                        .as_deref()
+                        .and_then(|p: &std::path::Path| p.file_stem())
+                        .map(|s: &std::ffi::OsStr| s.to_string_lossy().into_owned())
+                        .unwrap_or_else(|| "drawing".into());
+                    Task::perform(
+                        crate::io::fbx::pick_fbx_path_owned(stem),
+                        Message::FbxExportPath,
+                    )
+                }
+            }
+
+            Message::FbxExportPath(Some(path)) => self.on_fbx_export_path_some(path),
+
+            Message::FbxExportPath(None) => Task::none(),
+
+            Message::FbxExportFinished(path, result) => {
+                match result {
+                    Ok(()) => self.command_line.push_output(
+                        crate::tf!("FBXEXPORT: exported to \"{}\"", path.display()).as_ref(),
+                    ),
+                    Err(error) => self
+                        .command_line
+                        .push_error(crate::tf!("FBXEXPORT: {error}").as_ref()),
+                }
+                Task::none()
+            }
+
             Message::StepExportPath(Some(path)) => self.on_step_export_path_some(path),
 
             Message::StepExportPath(None) => Task::none(),