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/mod.rs

@@ -4411,6 +4411,12 @@ pub enum Message {
     /// Callback after the user picks (or cancels) the STEP save path.
     StepExportPath(Option<std::path::PathBuf>),
     StepExportFinished(std::path::PathBuf, Result<(), String>),
+    // ── FBX export ────────────────────────────────────────────────────────
+    /// Trigger FBX export: show save dialog.
+    FbxExport,
+    /// Callback after the user picks (or cancels) the FBX save path.
+    FbxExportPath(Option<std::path::PathBuf>),
+    FbxExportFinished(std::path::PathBuf, Result<(), String>),
     // ── OBJ import ────────────────────────────────────────────────────────
     /// Trigger OBJ import: show open-file dialog.
     ObjImport,