Cloud conversion now handles projects with large image libraries

Converting a browser project to a Cloud full-stack app no longer fails when the project carries a large media library.

Projects with many hundreds of images could get stuck at the import step with an unhelpful error, and retrying or restarting the project computer never helped. The importer now transfers big asset sets reliably, and if anything does go wrong during a conversion, the error now names the actual cause instead of a generic failure - so the AI can fix it or route it to us with real detail.