Align Node.js version across the repo (Node 24 default, CI on 24 + 26) #2465

Problem

The Node.js version is hard-coded in several places, and each one is updated in a separate PR. Some of them get left behind, which has already caused bugs:

• Docker setup doesn't work #2338: docker compose up failed at npm ci. The package-lock.json was regenerated by a local npm install on a newer Node/npm, and the Dockerfile's Node 22 rejected it as out of sync. Fixed by bumping the Dockerfile to Node 24 (Bump dockerfile node to v24 #2346).
• Add support for Node 22 #1970: npm install didn't work on newer Node. The fix (newer versions of node 14 and beyond are supported, modified package … #1999) had to update the Node version in the workflows, the Dockerfile and package.json at once.
• The engines bump to >=24 landed inside an unrelated Dependabot PR (build(deps): bump minimatch from 3.1.2 to 3.1.5 #2330), and the devcontainer was missed.

Current state on master (ee1bdee):

Some tools also enforce supported Node versions: after #2463, dependency-cruiser 18 refuses to run on Node 20 and 25, so npm run depcruise:* won't work in the current devcontainer (Node 20) or on a local Node 25.

Node 24 has been in maintenance LTS since 2026-10-20, and Node 26 became Active LTS on 2026-10-28 (release schedule). The plan is to stay on 24 for now and later migrate to 26, so 26 should already be supported and tested in CI.

Proposal

1. Add .nvmrc containing 24 as the default version (read by actions/setup-node, nvm use and fnm).
2. node.js.yml → build_and_test: run the matrix on [24.x, 26.x].
3. node.js.yml → deploy_gh_pages: use node-version-file: '.nvmrc' and move setup-node before npm install.
4. manual_test.yml: add 26.x to the node-version matrix.
5. Remove .devcontainer/ (and its entry in .prettierignore).
6. Dockerfile: keep node:24; optionally move it to ARG NODE_VERSION=24 so it's obvious what to update.
7. engines: change to "^24 || >=26", so 26 is listed as supported and odd, short-lived releases like 25 are excluded.

Done when

• .nvmrc exists; deploy_gh_pages reads it and installs dependencies after setup-node
• build_and_test passes on both 24.x and 26.x
• .devcontainer/ is removed
• Dockerfile and engines are consistent with the above