fix(basics): require Node 20.9+ to match Next.js 16

[Jeko92]

Fixes #1336

Next.js 16 (installed via next: latest) requires Node.js >=20.9.0, but all 10 basics/* starters still specify Node 18. On Node 18, next dev exits with:

You are using Node.js 18.20.8. For Next.js, Node.js version ">=20.9.0" is required.

Changes:

• .nvmrc: 18 → 24 (current LTS; Node 20 reached end-of-life in April 2026)
• engines.node: >=18 → >=20.9.0 (matches Next.js 16's own requirement)

The lesson text on https://nextjs.org/learn/pages-router/create-nextjs-app-setup still says Node 18. That needs a fix on the curriculum side.

Tested: pnpm test passes (lint + prettier).

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