Fix Export to CAM's default filename appending .stl instead of replacing the extension

tab_display_name() is the drawing's real filename including its own
extension (e.g. "Drawing1.dwg") - the CamExportDialogOpen handler
appended ".stl" directly onto that, producing "Drawing1.dwg.stl" instead
of "Drawing1.stl". Strips the existing extension via file_stem() first.

src/app/update/mod.rs

@@ -1182,7 +1182,15 @@ impl OpenCADStudio {
                         .push_error(crate::t!("STLOUT: no 3D mesh data in this drawing.").as_ref());
                     return Task::none();
                 }
-                self.cam_export_filename = format!("{}.stl", self.tabs[i].tab_display_name());
+                // tab_display_name() is the drawing's real filename including
+                // its own extension (e.g. "Drawing1.dwg") - appending ".stl"
+                // directly produced "Drawing1.dwg.stl" instead of replacing
+                // it (CMMS bug report, 2026-08-27).
+                let base = std::path::Path::new(&self.tabs[i].tab_display_name())
+                    .file_stem()
+                    .map(|s| s.to_string_lossy().into_owned())
+                    .unwrap_or_else(|| self.tabs[i].tab_display_name());
+                self.cam_export_filename = format!("{base}.stl");
                 self.active_modal = Some(crate::app::ModalKind::CamExportDialog);
                 Task::none()
             }