Background wallpaper not taken into account after restart (Linux appimage)

Issue type

UI bug (incorrect info or interface appearance)

Description with steps to reproduce

1. Choose a background (or paper) wallpaper via Preferences/appearances
2. Close MuseScore
3. The choice is saved in MuseScore.ini, e.g.
   canvas\foreground\wallpaper=/tmp/.mount_mscorenIIT9A/share/mscore4portable-4.4/wallpapers/paper5.png
   (note the hardcoded mountpoint)
4. Start MS: the chosen background is not applied. The appimage mountpoint is different, but the ini file still has the initial hardcoded mountpoint.

Supporting files, videos and screenshots

Discussed in https://musescore.org/en/4.4.3-rc

What is the latest version of MuseScore Studio where this issue is present?

4.4.3 nightly

Regression

Yes, this used to work in MuseScore 3.x and now is broken

Operating system

Ubuntu 22.04LTS

Additional context

No response

Checklist

• This report follows the guidelines for reporting bugs and issues
• I have verified that this issue has not been logged before, by searching the issue tracker for similar issues
• I have attached all requested files and information to this report
• I have attempted to identify the root problem as concisely as possible, and have used minimal reproducible examples where possible

Start by reproducing the restart flow on Ubuntu 22.04 with the Linux AppImage, then inspect how the canvas\foreground\wallpaper value is written to and read from MuseScore.ini. Done means a selected paper or background wallpaper is still applied after restarting when the AppImage mountpoint changes.