Save conflict protection

Every save now carries a version number. When your client saves to the cloud, it checks that no other session has written since your last load. If two devices save at the same time, or a background sync races against a server operation, the conflict is detected and the newer data wins. Previously, a slow save could silently overwrite a newer one. That can't happen anymore.

This matters most for enhancement, crafting, and shop transactions — operations where the server is the source of truth. The server bumps the version on every write, and your client tracks it. If the versions don't match, the client re-fetches the latest state instead of pushing stale data.