The Makefile hardcoded the Arch QML module dir (/usr/lib/qt6/qml), so
qmllint could not resolve org.kde.plasma.* / kirigami imports on the
Debian-based neon image and make check failed. Parametrize QML_IMPORT_DIR
(default unchanged for local Arch use) and have CI discover the real Qt6
QML dir via qtpaths6, falling back to the multiarch path.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>