fix: Use settings.set() instead of item assignment in _on_overlay_mode_changed

Settings object uses .set() method, not dictionary-style item assignment.
This fixes the TypeError when changing overlay mode in settings.

core/ui/settings_view.py

@@ -375,10 +375,9 @@ class SettingsView(QWidget):
         
         mode = mode_map.get(index, "overlay_game")
         
-        # Save to settings
+        # Save to settings using the proper set() method
         if hasattr(self.overlay, 'settings'):
-            self.overlay.settings['activity_bar.overlay_mode'] = mode
-            self.overlay.settings.save()
+            self.overlay.settings.set('activity_bar.overlay_mode', mode)
         
         print(f"[Settings] Activity Bar mode changed to: {mode}")