Add nodemon.json for full hot-reload (backend + frontend)

nodemon now watches both server.js and client/src/. On any change,
it rebuilds the React app and restarts Express — so git push makes
both backend AND frontend changes visible on the hot domain.

- dev script simplified to "nodemon" (reads nodemon.json config)
- dev:local added for human developers who want Vite HMR

package.json

@@ -5,9 +5,10 @@
   "main": "server.js",
   "scripts": {
     "start": "node server.js",
-    "dev": "concurrently \"nodemon server.js\" \"cd client && npm run dev\"",
+    "dev": "nodemon",
     "build": "cd client && npm run build",
     "postinstall": "[ -d client ] && cd client && npm install || true",
+    "dev:local": "concurrently \"nodemon server.js\" \"cd client && npm run dev\"",
     "dev:server": "nodemon server.js",
     "dev:client": "cd client && npm run dev"
   },