fix: replace gcloud shell-out with google-auth-library for Vertex AI tokens

gcloud is not available inside the Docker container. Use google-auth-library
instead, which reads credentials from the GCP metadata server (works on any
GCP VM) or GOOGLE_APPLICATION_CREDENTIALS env var. Also rebuilds dist/.

Made-with: Cursor

package.json

@@ -13,6 +13,7 @@
     "@google/genai": "^1.0.0",
     "cors": "^2.8.5",
     "express": "^4.19.2",
+    "google-auth-library": "^10.6.1",
     "minimatch": "^9.0.5",
     "uuid": "^9.0.1"
   },