[martastn]

Aligns the Node.js version across CI, Docker and package.json, and adds Node 26 to CI ahead of the migration.

Closes #2465

Changes:

• Add .nvmrc (24) as the single default version.
• node.js.yml:
  • build_and_test runs on [24.x, 26.x].
  • deploy_gh_pages reads the version from .nvmrc and runs setup-node before npm install. Previously it installed with the runner's preinstalled Node.
• manual_test.yml: add 26.x to the matrix.
• Dockerfile: ARG NODE_VERSION=24 in place of the hard-coded node:24.
• engines: >=24.0.0 → ^24 || >=26, which excludes odd releases like 25.
• Remove the unused .devcontainer/, which was still on Node 20.

[grzanka]

@martastn check also if technical documentation requires update: https://yaptide.github.io/for_developers/local-setup/local-frontend-demo/#prerequisites

[Copilot]

🟢 Approval recommended

The review surfaced no concrete defects or outstanding issues across the scoped changes, leaving nothing requiring further action before approval.

No open issues 🎉

What changed in this PR

This PR consolidates the Node.js version into a single source of truth to prevent the version drift that previously caused build failures (per issue #2465). It introduces .nvmrc as the default (Node 24), expands CI to also test Node 26 ahead of the planned migration, parameterizes the Dockerfile, broadens the engines range, and removes the stale (Node 20) devcontainer.

Changes:

• Add .nvmrc (24) and have deploy_gh_pages read it via node-version-file, with setup-node now correctly running before npm install.
• Add 26.x to the CI matrices in node.js.yml and manual_test.yml, parameterize the Dockerfile with ARG NODE_VERSION=24, and widen engines to ^24 || >=26.
• Remove the unused .devcontainer/ and its .prettierignore entry.

File	Description
.nvmrc	New single source of truth for the default Node version (24).
.github/​workflows/​node.js.yml	Matrix extended to [24.x, 26.x]; deploy_gh_pages now installs Node from .nvmrc before npm install.
.github/​workflows/​manual_test.yml	Adds 26.x to the on-demand test matrix.
Dockerfile	Replaces hard-coded node:24 with ARG NODE_VERSION=24 + FROM node:${NODE_VERSION}.
package.json	engines.node widened to `^24
package-lock.json	Mirrors the updated engines range to stay in sync.
.prettierignore	Removes the now-obsolete .devcontainer/ ignore entry.
.devcontainer/​devcontainer.json	Removed; it was pinned to the outdated Node 20 image and unused.

_{Review effort: Balanced}

[martastn]

@martastn check also if technical documentation requires update: https://yaptide.github.io/for_developers/local-setup/local-frontend-demo/#prerequisites

I updated the documentation, here's the PR: yaptide/for_developers#102