Ash Of Sin: Auth

Merges TrueUUID's premium session check and offlineauth's password login into a single login flow: premium players get in seamlessly, everyone else claims their name and save via /ashofsin auth register|login. Once a name is bound to a premium account, offline impersonation is refused. identity-overrides.json provides force-offline / no-auth (bots) / force-premium lists with case-sensitive name matching, while unlisted names keep the automatic premium detection. Existing offline saves are copied to the premium UUID (source kept, no inventory transplant).