CI: bump actions to Node.js 24-compatible versions

[zolizoli]

GitHub deprecated Node.js 20 runners. The following actions emit a warning on every CI run:

• `actions/checkout@v4`
• `astral-sh/setup-uv@v4`

Deadlines (from GitHub):

• June 2, 2026 — runners forced to Node.js 24
• September 16, 2026 — Node.js 20 removed

Affected workflows:

• `.github/workflows/ci.yml`
• `.github/workflows/publish.yml`

Action: bump to the current major versions that ship with Node.js 24 support, or set `FORCE_JAVASCRIPT_ACTIONS_TO_NODE24=true` as a stopgap.

Non-urgent — CI still works, just warns.