ci: read the setup-node version from .nvmrc

Optional tidy-up, left open by #445: let the remaining setup-node steps read .nvmrc so every Node in CI is the pinned one.

#445 pinned the Node that builds the theme's assets to 24.21.0 ([tool.sphinx-theme-builder] node-version in pyproject.toml) and set .nvmrc to the same exact version. The workflows' own setup-node steps still ask for any 24.x:

None of them compiles an asset that ships: visual and update-snapshots.yml run npm ci and Playwright, and in docs.yml the editable install after npm run build recompiles the assets on sphinx-theme-builder's own Node. But '24' resolves to whatever 24.x the runner has cached, 24.20.0 at the time of #445, so Playwright runs on a different Node from the build.

QuantEcon/quantecon-sphinx-theme#9 switched its docs.yml to node-version-file: .nvmrc after review. Doing the same here would keep all three in step with one file. docs.yml runs only on pushes to main and update-snapshots.yml only from the default branch, so a PR's CI cannot fully exercise the change; the merge run is its first real test.

Tracked from QuantEcon/workspace-themes#31 (closed 2026-09-19). Low priority.

🤖 Generated with Claude Code

Start by reading the four setup-node entries identified in .github/workflows/ci.yml, .github/workflows/docs.yml, and .github/workflows/update-snapshots.yml, then compare them with .nvmrc and the existing pyproject.toml Node pin. Check the workflow syntax and confirm that all remaining CI Node setup uses the version from .nvmrc; the docs and snapshot workflows receive their first full validation after merging to the default branch.