codestyle: explain why we instantiate the app at module level

src/teilchensammler_cli/main.py

@@ -20,9 +20,10 @@ logger = logging.getLogger(__name__)
 class SammlerApp(App):
     async def on_mount(self) -> None:
         create_db_and_tables()
-        _ = self.push_screen(AddInventoryScreen())
+        self.push_screen(AddInventoryScreen())
 
 
+# so we can import it without running it
 app = SammlerApp()