choredocker): enhance local Docker socket access and error handling

- Added guidance for Docker socket group access in docker-compose files.
- Introduced docker-compose.override.example.yml for supplemental group configuration.
- Improved entrypoint diagnostics to include socket GID and group guidance.
- Updated README with instructions for setting up Docker socket access.
- Enhanced backend error handling to provide actionable messages for permission issues.
- Updated frontend components to display troubleshooting information regarding Docker socket access.
- Added tests to ensure proper error messages and guidance are rendered in UI.
- Revised code coverage settings to include Docker service files for better regression tracking.

codecov.yml

@@ -74,10 +74,6 @@ ignore:
   - "backend/*.html"
   - "backend/codeql-db/**"
 
-  # Docker-only code (not testable in CI)
-  - "backend/internal/services/docker_service.go"
-  - "backend/internal/api/handlers/docker_handler.go"
-
   # CodeQL artifacts
   - "codeql-db/**"
   - "codeql-db-*/**"