Export Frontend: work around NextJS aborting on the nodejs API.

This introduces a pre-build step on Next Build, which hides the files
in the app/api directory when the EXPORT_FRONTEND environment
variable is true-ish.

Hopefully there won't be disruption due to the post-processing step.

Also check https://github.com/vercel/next.js/issues/61213 for
upstream updates.

.gitignore

@@ -1,5 +1,8 @@
 # See https://help.github.com/articles/ignoring-files/ for more about ignoring files.
 
+# Frontend Build: ignore API files disabled for this build
+/app/**/*.backup
+
 # dependencies
 /node_modules
 /.pnp