fix(ui): add @types/node for process.env in vite.config

tsc -b type-checks vite.config.ts during the production build (pnpm
lint does not), and the API_PROXY_TARGET override added there needs
node types for `process`.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

ui/package.json

@@ -25,6 +25,7 @@
     "remark-gfm": "^4.0.1"
   },
   "devDependencies": {
+    "@types/node": "^25.9.3",
     "@types/react": "^19.0.0",
     "@types/react-dom": "^19.0.0",
     "@types/react-vertical-timeline-component": "^3.3.6",