fix: add Node >= 20 engine requirement to frontend package.json

Tailwind CSS v4.2 (@tailwindcss/oxide) requires Node >= 20 for native
bindings. Without this, `npm install` succeeds but `vite` fails at
runtime with a missing native binding error.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/package.json

@@ -3,6 +3,9 @@
   "private": true,
   "version": "0.1.0",
   "type": "module",
+  "engines": {
+    "node": ">=20"
+  },
   "scripts": {
     "dev": "vite",
     "build": "tsc -b && vite build",