Add GUI runner and packaging docs

src/run_gui.py

@@ -0,0 +1,4 @@
+from seedpass_gui.app import main
+
+if __name__ == "__main__":
+    main()

src/seedpass_gui/app.py

@@ -197,3 +197,8 @@ class SeedPassApp(toga.App):
         )
         self.main_window = None
         self.lock_window.show()
+
+
+def main() -> None:  # pragma: no cover - GUI bootstrap
+    """Run the BeeWare application."""
+    SeedPassApp().main_loop()