chore: bump minimum Node.js version from 20.0 to 20.19

[yangshun]

Summary

• Bump minimum Node.js version from 20.0 to 20.19 across the entire repo
• Oxfmt requires Node.js >= 20.19.0, which was causing CI failures on the 20.0 matrix entry

I'm bumping to 20.19 first as the minimum version so we can proceed with other tooling upgrade work

Changes

• CI workflows: Updated Node matrix entry 20.0 → 20.19 in tests.yml, tests-e2e.yml, tests-windows.yml
• Documentation: Updated version references in installation.mdx, example intro.md files, and template docs
• Package engines: Updated "node": ">=20.0" → ">=20.19" in all 40 package.json files

Test plan

• CI passes on all Node version matrix entries
• Verify grep -r '">=20.0"' --include='package.json' returns no results

🤖 Generated with Claude Code

[socket-security]

Review the following changes in direct dependencies. Learn more about Socket for GitHub.

Diff	Package	Supply Chain Security	Vulnerability	Quality	Maintenance	License
	@​docusaurus/​tsconfig@​3.9.2
	@​docusaurus/​module-type-aliases@​3.9.2
	@​docusaurus/​preset-classic@​3.9.2
	@​docusaurus/​types@​3.9.2
	@​docusaurus/​core@​3.9.2
	react@​19.2.4 ⏵ 19.2.0
	@​mdx-js/​react@​1.0.1 ⏵ 3.1.1	+1		+15	+3
	prism-react-renderer@​2.4.1
	react-dom@​19.2.4 ⏵ 19.2.0

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[socket-security]

Warning

Review the following alerts detected in dependencies.

According to your organization's Security Policy, it is recommended to resolve "Warn" alerts. Learn more about Socket for GitHub.

Action	Severity	Alert  (click "▶" to expand/collapse)
Warn		Obfuscated code: npm entities is 91.0% likely obfuscated Confidence: 0.91 Location: Package overview From: ? → npm/@docusaurus/preset-classic@3.9.2 → npm/@docusaurus/core@3.9.2 → npm/entities@4.5.0 ℹ Read more on: This package | This alert | What is obfuscated code? Next steps: Take a moment to review the security alert above. Review the linked package source code to understand the potential risk. Ensure the package is not malicious before proceeding. If you're unsure how to proceed, reach out to your security team or ask the Socket team for help at support@socket.dev. Suggestion: Packages should not obfuscate their code. Consider not using packages with obfuscated code. Mark the package as acceptable risk. To ignore this alert only in this pull request, reply with the comment @SocketSecurity ignore npm/entities@4.5.0. You can also ignore all packages with @SocketSecurity ignore-all. To ignore an alert for all future pull requests, use Socket's Dashboard to change the triage state of this alert.
Warn		Obfuscated code: npm entities is 91.0% likely obfuscated Confidence: 0.91 Location: Package overview From: ? → npm/@docusaurus/preset-classic@3.9.2 → npm/@docusaurus/core@3.9.2 → npm/entities@6.0.1 ℹ Read more on: This package | This alert | What is obfuscated code? Next steps: Take a moment to review the security alert above. Review the linked package source code to understand the potential risk. Ensure the package is not malicious before proceeding. If you're unsure how to proceed, reach out to your security team or ask the Socket team for help at support@socket.dev. Suggestion: Packages should not obfuscate their code. Consider not using packages with obfuscated code. Mark the package as acceptable risk. To ignore this alert only in this pull request, reply with the comment @SocketSecurity ignore npm/entities@6.0.1. You can also ignore all packages with @SocketSecurity ignore-all. To ignore an alert for all future pull requests, use Socket's Dashboard to change the triage state of this alert.

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[yangshun]

Not sure what the Lighthouse CI failure is about