ci: run qmltestrunner with offscreen Qt platform

Headless CI runner has no X display, so qmltest aborts loading the
xcb plugin. Set QT_QPA_PLATFORM=offscreen for the ctest invocation
in CI and in both Makefile qmltest targets so local headless runs
work too.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitea/workflows/ci.yml

@@ -79,6 +79,8 @@ jobs:
 
       - name: QML unit tests (qmltestrunner via Qt::QuickTest)
         working-directory: framework/qml
+        env:
+          QT_QPA_PLATFORM: offscreen
         run: |
           cmake -S . -B build-tests -DBUILD_TESTING=ON
           cmake --build build-tests --target qml_unit_tests --parallel