Set QT_QPA_PLATFORM=offscreen for both grgsm_livemon and
grgsm_scanner to prevent SIGABRT when no X11 display is available.
grgsm_livemon uses GNU Radio which loads Qt plugins — without a
display, Qt aborts with "could not connect to display".
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>