Fix: Resolve frontend build failure on Node 17+

[cattailsswing]

This PR fixes a build failure for the frontend static site (render-todo) when deploying on modern Node.js versions (v17 and higher).

The Problem

The project uses react-scripts@4.0.3, which relies on an old crypto algorithm (MD4). Modern Node.js versions have disabled this algorithm for security reasons, causing the build to fail with an ERR_OSSL_EVP_UNSUPPORTED error.

The Solution

This PR makes two changes to the frontend/package.json file to fix the build:

1. Sets Node.js version: Adds an engines field to pin the Node.js version to 18.x, which is a compatible LTS release.

   "engines": {
     "node": "18.x.x"
   },

2. Enables legacy crypto: Adds the NODE_OPTIONS=--openssl-legacy-provider flag to the build script. This tells Node 18 to load the old crypto library that react-scripts@4.0.3 requires to run.

   "build": "NODE_OPTIONS=--openssl-legacy-provider react-scripts build",

These changes allow the project to build successfully on Render (as of Oct 22, 2025).