feat(about): version chip + About dialog + ghcr image publish

[jamofer]

Summary

• New GET /api/v1/about returns {version, ref, commit, built_at} from env vars baked into the image at build time. Public, unauthenticated.
• Backend Dockerfiles accept GIT_REF / GIT_COMMIT / BUILD_DATE build-args → injected as TESTJAM_VERSION_* env. Dev stack derives them from git on every make up so a branch checkout flows into the running container without a rebuild.
• Sidebar shows a clickable version chip just under the logo (v1.0.0 for tagged builds, <branch> @ <sha> for dev). Click — or About in the user dropdown — opens a dialog with full metadata.
• Dialog respects the user's timezone preference and the browser's locale for the build date.
• i18n: new about namespace + user.about key in all five locales (en/es/ca/gl/eu).
• New .github/workflows/publish-images.yml builds backend/Dockerfile.prod and frontend/Dockerfile.prod on tag v* (or manual dispatch) and pushes to ghcr.io/<owner>/testjam-api:{tag,latest} and ghcr.io/<owner>/testjam-web:{tag,latest} with the same GIT_REF / GIT_COMMIT / BUILD_DATE build-args wired through.