chore: test maintained Node.js releases

[sadjow]

Summary

• test the latest patch releases of Node.js 22, 24, and 26 in CI
• use the latest Node.js 24 release for local development
• use Node.js 24 for npm publishing workflows
• keep the package free of an engines restriction so other Node.js versions can still be tried on a best-effort basis

The CI matrix defines the major versions the project actively verifies; it does not block installation on other versions.

Validation

• root and playground frozen-lockfile installs
• yarn lint --max-warnings 0
• all 53 tests on Node.js 22.22, 24.13, and 26.7
• Nuxt production builds on Node.js 22.22, 24.13, and 26.7
• standalone playground build
• npm pack --dry-run on Node.js 24.13

Existing issue

yarn test:types continues to report existing strict-nullability errors in runtime and test files. This PR does not change TypeScript sources, and the command is not part of the current CI workflow.