fix: improve npm install error visibility in build-vercel.sh

Agent-Logs-Url: https://github.com/objectstack-ai/framework/sessions/a2378bd5-dff3-42d5-bf08-1dff787cf351

Co-authored-by: xuyushun441-sys <255036401+xuyushun441-sys@users.noreply.github.com>

Author: Copilot

apps/server/scripts/build-vercel.sh

@@ -55,7 +55,7 @@ cat > api/_package.json << 'DEPS'
 DEPS
 cd api
 mv _package.json package.json
-npm install --production --no-package-lock --ignore-scripts 2>&1 | tail -3
+npm install --production --no-package-lock --ignore-scripts --loglevel error
 rm package.json
 cd ..
 echo "[build-vercel]   ✓ External dependencies installed in api/node_modules/"