feat: Set units for viewer spawned from the view menu (#1308)

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-->

## Summary by CodeRabbit

* **New Features**
* The viewer’s scale bar now defaults to nanometers (nm), providing
clearer measurement context for high-resolution imagery.
* Enhances consistency of units across sessions, reducing confusion when
analyzing microscopy data.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Author: Czaki

package/PartSeg/common_gui/main_window.py

@@ -228,6 +228,7 @@ def get_colormaps(self) -> list[Optional[colormap.Colormap]]:
 
     def napari_viewer_show(self):
         viewer = Viewer(title="Additional output", settings=self.settings, partseg_viewer_name=self.channel_info)
+        viewer.scale_bar.unit = "nm"
         viewer.theme = self.settings.theme_name
         viewer.create_initial_layers(image=True, roi=True, additional_layers=False, points=True)
         self.viewer_list.append(viewer)