imagej fails to read/write ImageJ.cfg in a user-writable location when installed system wide

As a HPC system administrator, I have imagej installed in a module system (https://lmod.readthedocs.io/en/latest/) in a read-only location.

When a user attempts to change a setting, they are presented with this:
because of course the installation location is read-only for them.

ImageJ should honour Unix/Linux standards of placing configs either in `$HOME/.imagej` or even better the XDG standard, https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html which is configurable, but defaults to `$HOME/.confg` for config files.

Locate the code that reads and writes ImageJ.cfg, then compare its current installation-location behavior with the requested $HOME/.imagej and XDG-based locations. Done means users can change settings when ImageJ is installed read-only, with the configuration stored in a user-writable location.

Written by the indexing model from the issue text.