Update training

src/gui/main_window.py

@@ -297,7 +297,9 @@ class MainWindow(QMainWindow):
             # Save window state before closing
             self._save_window_state()
 
-            # Save annotation tab state if it exists
+            # Persist tab state and stop background work before exit
+            if hasattr(self, "training_tab"):
+                self.training_tab.shutdown()
             if hasattr(self, "annotation_tab"):
                 self.annotation_tab.save_state()