Add Qt preview window launcher

[Greensand321]

Motivation

• Provide a simple standalone Qt window to begin an incremental Qt migration and allow new Qt panels to be developed alongside the existing Tkinter UI.
• Make the Qt preview optional and discoverable from the existing Tools menu to let users try Qt functionality without embedding it directly in the Tk app.
• Add an explicit optional dependency so developers can install Qt support when desired.

Description

• Added a new lightweight Qt launcher module at gui/qt_launcher.py that prefers PySide6 and falls back to PyQt6, and implements a QtPreviewWindow with placeholder content.
• Added an empty package file gui/__init__.py and added PySide6 to requirements.txt as an optional UX dependency.
• Added an _open_qt_preview_window handler to main_gui.py that checks for PySide6/PyQt6 via importlib.util.find_spec, shows an error if missing, and launches the Qt process with subprocess.Popen([sys.executable, "-m", "gui.qt_launcher"]) when available.
• Added a Tools menu entry labeled Qt Preview Window… in main_gui.py to invoke the new handler.

Testing

• No automated tests were run for this change (not requested).

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Codex Task