Create new shared package for Better Auth type definitions:
- UserRole, JWTPayload, CurrentUserData types
- Type guards: isValidUserRole, isValidOrganizationRole
- Utility functions: jwtPayloadToCurrentUser
- userAdditionalFields for client type inference
Migrate shared packages to use centralized types:
- @manacore/shared-auth re-exports from new package
- @manacore/shared-nestjs-auth uses new package as dependency
