Add Windows build script and update README with build instructions

build_win.cmd

@@ -0,0 +1,43 @@
+@echo off
+chcp 65001 >nul
+setlocal
+
+set "PY=.venv\Scripts\python.exe"
+set "DIST=build\onefile"
+
+if not exist "%DIST%" mkdir "%DIST%"
+
+"%PY%" -m nuitka ^
+  --mode=onefile ^
+  --msvc=latest ^
+  --enable-plugin=pyside6 ^
+  --include-qt-plugins=platforms,imageformats,styles,iconengines ^
+  --windows-console-mode=disable ^
+  --output-dir="%DIST%" ^
+  --output-filename=a1packer.exe ^
+  --remove-output ^
+  run_gui.py
+if errorlevel 1 exit /b %ERRORLEVEL%
+
+echo.
+echo GUI build done.
+echo Next step is building CLI.
+
+"%PY%" -m nuitka ^
+  --mode=onefile ^
+  --msvc=latest ^
+  --output-dir="%DIST%" ^
+  --output-filename=a1packer-cli.exe ^
+  --remove-output ^
+  run_cli.py
+if errorlevel 1 exit /b %ERRORLEVEL%
+
+robocopy "swap_gcode" "%DIST%\swap_gcode" /E
+if %ERRORLEVEL% GEQ 8 exit /b %ERRORLEVEL%
+
+copy /Y "gcode_patches.ini" "%DIST%\gcode_patches.ini" >nul
+if errorlevel 1 exit /b %ERRORLEVEL%
+
+echo.
+echo Build done: %DIST%
+exit /b 0