refactor: simplify setup flow — eliminate redundant DB calls and login round-trips

- Add AdminExists() to UserRepository (EXISTS query, not full table scan)
- SetupInitialAdmin returns tokens directly (skip separate Login call)
- Add SetupRequest DTO to auth_dto.go (replace inline struct)
- Extract defaultEmail() helper (removes duplicated email logic)
- AuthPage uses setup tokens directly (skip redundant apiLogin call)
- Use customAxiosInstance for auth API calls (consistent with codebase)

backend/internal/domain/repository/user_repository.go

@@ -24,4 +24,7 @@ type UserRepository interface {
 
 	// List 列出所有用户
 	List(ctx context.Context) ([]*entity.User, error)
+
+	// AdminExists checks whether any admin user exists (lightweight EXISTS query)
+	AdminExists(ctx context.Context) (bool, error)
 }