docs: correct Node.js prerequisite for Next.js 16

[Mohamedajab]

Description

Correct the outdated Node.js 18+ prerequisite in both README.md and CONTRIBUTING.md to Node.js 20.9.0+, preserving Bun as the recommended option. Note that Next.js 16 no longer supports Node.js 18 so the two setup guides agree.

Related Issue

Closes #1776 (assigned to Mohamedajab).

Type of Contribution

• Documentation update
• GSSoC contribution

Participant Info

• GitHub username: Mohamedajab
• Contribution level: Beginner

Validation

• npm view next@16.3.4 engines --json reports node >=20.9.0 for the exact package version pinned here.
• Cross-checked the official Next.js 16 requirements: https://nextjs.org/blog/next-16
• git diff --check passed.
• Docs only; application lint, typecheck and builds were not run. No UI changes, so a screen recording is not applicable.

Checklist

• Read the contribution guidelines
• Kept setup guides consistent
• Linked the assigned issue
• No runtime or dependency changes

[netlify]

👷 Deploy request for reframe-os pending review.

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🔨 Latest commit	671ce9b

[github-actions]

👋 Thanks for your PR, @Mohamedajab!

Welcome to Reframe — a browser-based video editor built for everyone 🎬

🟠 GSSoC'26 PR detected — thanks for contributing under GirlScript Summer of Code 2026!

What happens next

1. 🤖 Automated checks — build & TypeScript typecheck will run automatically
2. ✅ Vercel preview — a preview deployment will be created (requires maintainer authorization for fork PRs)
3. 👀 Code review — a maintainer will review your changes
4. 🚀 Merge — once approved, your PR will be merged!

Quick checklist

• PR title follows Conventional Commits (e.g. feat: add dark mode)
• Linked the issue this PR closes (e.g. Closes #123)
• Tested the changes locally (bun run dev)
• Build passes (bun run build)

Useful links

• 📖 CONTRIBUTING.md
• 🐛 Open Issues
• ⭐ Star the repo to support the project!

Happy coding! 🎉

[github-actions]

✅ PR Format Check Passed — @Mohamedajab

Basic format checks passed. A maintainer will review your code changes.

This does not mean the PR is approved — it just means the format is correct.