fix(workflow): properly propagate parallel build errors on Vercel

`wait` without arguments returns only the last process's exit status,
so a failure in the first background build was silently swallowed.
Capture PIDs and wait on each explicitly to ensure any failure blocks
the Vercel deployment.

build.sh

@@ -101,9 +101,10 @@ main() {
 
   # Build the site
   echo "Building the site..."
-  build demo &
+  build demo & pid_demo=$!
-  build test &
+  build test & pid_test=$!
-  wait
+  wait "${pid_demo}" || { echo "build demo failed"; exit 1; }
+  wait "${pid_test}" || { echo "build test failed"; exit 1; }
   pnpm -F integration start
 }