feat: Add LCOV coverage support for Jenkins CI

- Add ENABLE_COVERAGE CMake option with gcov flags
- Create collect-coverage.sh script for LCOV workflow
- Add Qt6 test files with QTest framework
- Update Jenkinsfile with Coverage stage (main/master/coverage.*)
- Update CI Docker image with lcov and genhtml packages
- Add .gitignore entries for coverage artifacts
- Add COVERAGE.md documentation

Coverage workflow:
1. Configure with -DENABLE_COVERAGE=ON
2. Build and run tests to generate .gcda/.gcno files
3. Collect coverage with lcov
4. Filter out Qt/system libraries
5. Generate HTML report and lcov.info for Jenkins

Jenkins publishes coverage/lcov.info using Coverage Plugin.
Coverage stage only runs on protected branches.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Entire-Checkpoint: 7783c7698334
Entire-Checkpoint: 70341e29996b
Signed-off-by: twi-devel <torsten.wierschin@life-maps.com>

ci/Dockerfile.ci

@@ -18,7 +18,9 @@ RUN apt-get update && \
       qt6-base-dev \
       qt6-base-dev-tools \
       qt6-tools-dev \
-      qt6-tools-dev-tools && \
+      qt6-tools-dev-tools \
+      lcov \
+      genhtml && \
     rm -rf /var/lib/apt/lists/*
 
 # Set Qt platform for headless testing