Modernize tooling baseline for release validation

[mehul0810]

Summary

• Add Node 24 project policy with local version files, engines, and CI usage.
• Add PHPUnit, frontend, and Playwright smoke workflows, plus Dependabot coverage for npm, Composer, and GitHub Actions.
• Make JS/CSS lint, build, PHPStan, PHPUnit, and package zip validation reproducible on the release branch.
• Modernize release/prerelease workflows to maintained action versions and remove legacy Node 20/sudo build usage.

Validation

• composer validate --no-check-publish
• composer lint
• composer test
• composer phpstan
• npm ci
• npm run lint
• npm run build
• npm run plugin-zip with production Composer dependencies
• npx playwright test --list
• GitHub workflow YAML parse check

Notes

• Full wp-env Playwright execution was not run locally because Docker is not installed in this environment; the new GitHub Actions workflow runs it in CI.
• npm audit still reports existing dev dependency advisories in the current dependency tree; Dependabot is now configured to keep that work moving.

Closes #86

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