Rename brewpi_win.py to main_window.py

Pure rename of the pyuic5-generated UI module plus its one import
site in brewpi_gui.py; regenerated from brewpi.ui, no content changes.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

client/brewpi_gui.py

@@ -1,5 +1,5 @@
 #!/usr/bin/env python3
-from brewpi_win import Ui_MainWindow
+from main_window import Ui_MainWindow
 from PyQt5 import QtWidgets, QtGui
 import sys
 from ws.client.ws_client import WsClient