feat: add unified dev script that launches server + client

npm run dev now starts both the ERP proxy server and Vite in parallel.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

package.json

@@ -18,6 +18,9 @@
     "web-vitals": "^2.1.4"
   },
   "scripts": {
+    "dev": "concurrently -n server,client -c blue,green \"npm run dev:server\" \"npm run dev:client\"",
+    "dev:client": "vite",
+    "dev:server": "cd server && npm run dev",
     "start": "vite",
     "build": "vite build",
     "preview": "vite preview"
@@ -28,6 +31,7 @@
     "@types/react-dom": "^19.2.3",
     "@types/react-router-dom": "^5.3.3",
     "@vitejs/plugin-react": "^5.1.4",
+    "concurrently": "^9.2.1",
     "typescript": "^5.9.3",
     "vite": "^7.3.1"
   }