dx: add Node.js and npm version check to install.sh

[George-iam]

The install script (install.sh) runs npm install and npm run build but does not verify that Node.js >= 20 is installed, which is required by the engines field in package.json. If a user runs the script with Node.js 18 or older, they get a cryptic build error instead of a clear message. Add an early version check that prints a helpful error and exits if the Node.js version is below 20.

Relevant files:

• install.sh — add a version check near the top, before npm install
• package.json — reference for the engines.node requirement (>=20)

Acceptance criteria:

• Running install.sh with Node.js < 20 prints a clear error message (e.g., "Error: Node.js >= 20 is required. Found: v18.x.x") and exits with code 1
• Running with Node.js >= 20 proceeds normally
• The check works on both macOS and Linux