CI on Node 24 actions; Node.js 26 for CI and the frontend image

[k3mpaxl]

Why

Every job showed the annotation "Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24 …". That covered checkout, setup-python, setup-node and all docker/* actions in CI and in the Docker publish workflow.

What changed

• Actions on their current major versions, which all run on Node 24:

  Action	Before	After
  actions/checkout	v4	v7
  actions/setup-python	v5	v7
  actions/setup-node	v4	v7
  actions/upload-artifact	v4	v7
  docker/setup-buildx-action	v3	v4
  docker/setup-qemu-action	v3	v4
  docker/login-action	v3	v4
  docker/build-push-action	v6	v7
  docker/metadata-action	v5	v6

• Node.js 26 for CI (node-version: '26') and for the frontend stage of the Docker image (node:26-alpine, available for amd64 and arm64), instead of 22. 26.10.0 is the current release; the 26 line becomes LTS at the end of October 2026.

• README: the documented minimum stays Node.js 20+ (Vite and Playwright accept 20+); it now notes that the image and CI use Python 3.14 and Node.js 26.

• The runner stays ubuntu-latest. The notice that it moves to Ubuntu 26 from 19 October 2026 remains until GitHub switches.

For reviewers

• The release notes of every skipped major were read: Node 24 runtime; checkout v6 stores credentials in a separate file and v7 blocks fork checkouts for pull_request_target; setup-node v6 limits automatic caching to npm (cache: npm is set explicitly); setup-python v7 drops pip-install; upload-artifact v7 adds opt-in direct uploads; the docker majors remove deprecated inputs and env vars. None of these is used here.
• Every input the workflows pass was checked against the new action.yml files; none is missing.
• The actions' own runtime (Node 24) is chosen by the action authors and cannot be set to 26 from the workflow.
• The Docker publish workflow only runs on tags, so its annotations clear with the next release; the docker-build job of this PR builds the image with node:26-alpine.
• Locally the web tests and builds already run on Node 26.8.1.

🤖 Generated with Claude Code