Replace string-based role handling with a typed UserRole enum. Adds src/entities/enums/user_role.dart and exports it from krow_domain. Update SessionHandlerMixin to use List<UserRole> and change fetchUserRole to return UserRole?. V2SessionService now derives the role via UserRole.fromSessionData, and client/staff SessionListener widgets pass const <UserRole>[...] when initializing the auth listener. This centralizes role derivation and eliminates scattered role string literals.
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