CI(linux): run tests with QT_QPA_PLATFORM=offscreen

GitHub runner has no X display, so Qt Test binaries that pull in
QGuiApplication abort with "could not connect to display". The
offscreen platform plugin is shipped with qt6-base-dev and lets the
tests run headless.

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

Author: Quadstronaut

.github/workflows/build-linux.yml

@@ -28,6 +28,8 @@ jobs:
         run: cmake --build build
 
       - name: Test
+        env:
+          QT_QPA_PLATFORM: offscreen
         run: ctest --test-dir build --output-on-failure
 
       - name: Upload binary artifact