feat: STL export (STLOUT / EXPORTSTL command)

Exports all tessellated 3D mesh models in the current drawing to a
binary STL file via a save-file dialog. Normals are taken from the
MeshModel where available, otherwise computed from triangle vertices.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/app/commands.rs

@@ -3185,6 +3185,11 @@ impl H7CAD {
                 }
             }
 
+            // ── STL export ────────────────────────────────────────────────
+            "STLOUT"|"EXPORTSTL" => {
+                return Task::done(Message::StlExport);
+            }
+
             // ── Plot / Page Setup ──────────────────────────────────────────
             "PRINT"|"PLOT"|"EXPORT" => {
                 return Task::done(Message::PlotExport);