fix(ci): bump Node to 22 for Vite 8 (dist rebuild workflow + Space Dockerfile)

[cn0303]

What does this PR do?

A Dependabot PR bumped the repo to Vite 8 (build(deps-dev): bump vite 5→8),
which requires Node ^20.19.0 || >=22.12.0. Two build pipelines were still
pinned to Node 18, so npm run build fails on both. This bumps both to
Node 22.

The two breakages (same root cause)

1. .github/workflows/build_frontend.yml — the "Rebuild frontend/dist"
workflow. On Node 18 its build failed silently (a stale generated file, not a
red ❌), so the committed frontend/dist stopped being regenerated and the
shipped bundle fell behind the merged source — e.g. the PR #21 calibration
UI never reached dist, so a plain pip install / lelab run still served the
old UI.

2. frontend/Dockerfile — the image the Hugging Face Space builds
(FROM node:18-alpine). Its npm run build step fails, so the Space currently
shows a build error and the live demo is down.

⚠️ Worth flagging: this is a Dependabot blind spot — the bot bumped the
build tool's Node requirement, but nothing bumped the node-version in CI or
the node:18-alpine base image, and the failures are easy to miss (a stale
file, and a Space build error on a separate runtime). It may be worth having
Dependabot also watch actions/setup-node / the Dockerfile base image so a
future build-tool bump can't silently desync these again.

Notes

• The rebuild workflow also triggers on changes to its own file, so merging
  this re-runs it on Node 22 and regenerates frontend/dist from the current
  source — no hand-built dist is committed here.
• Verified the Vite 8 build passes on Node ≥20 locally; the Dockerfile change is
  the base-image tag only.

[nicolas-rabault]

Thank you,
You fix things faster than I catch the bugs 🤗.

[cn0303]

Thanks a lot!!