Loading models progress button is dark in Slicer light mode

[JonaRichter]

The "Loading models…" progress button shown during Run Analysis uses a hardcoded dark stylesheet (dark background, white text in _mrq_chunk/_mrq_base in widget.py). In Slicer's light theme this renders as a dark block that clashes with the surrounding UI.

Make the progress button respect the active Slicer palette (or provide a light-mode variant) so it looks correct in both light and dark themes.