CI: migrate GitHub Actions to Node.js 24 before June 2 deadline

[11me]

Node.js 20 actions are deprecated and will be forced to Node.js 24 starting June 2, 2026.

Actions to update

Action	Current	Fix	Effort
actions/checkout	v4	Upgrade to v6	trivial
actions/setup-node	v4	Upgrade to v6	trivial
softprops/action-gh-release	v2	Replace with gh release create	small
browser-actions/release-firefox-addon	v0.2.1	Replace with web-ext sign	medium
browser-actions/release-chrome-extension	v0.2.1	Replace with Chrome Web Store API	medium

Notes

• checkout/setup-node are drop-in upgrades
• browser-actions have no Node 24 release planned — need to rewrite as run steps using official CLIs
• softprops/action-gh-release can be replaced with gh release create + gh release upload

Ref

https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/