Add PyInstaller packaging (--onedir) as alternative to pyw launch

Avoids the cmd.exe flash from double-clicking run_desktop.bat and
gives a proper double-click .exe like VersaGUI.exe. Must be deployed
locally (%LOCALAPPDATA%) -- Windows silently refuses to load the
_internal DLLs when the .exe sits on the Z: network share, no error
shown. build_and_deploy.ps1 handles build + local copy in one step.

.gitignore

@@ -1,3 +1,5 @@
 __pycache__/
 *.pyc
 *.log
+build/
+dist/