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/adapter/output/persistence/postgres/user_repository.go

@@ -120,6 +120,12 @@ func (r *UserRepository) GetByUsername(ctx context.Context, username string) (*e
 }
 
 // Update 更新用户
+func (r *UserRepository) AdminExists(ctx context.Context) (bool, error) {
+	var exists bool
+	err := r.db.conn.QueryRowContext(ctx, `SELECT EXISTS(SELECT 1 FROM users WHERE role = 'admin')`).Scan(&exists)
+	return exists, err
+}
+
 func (r *UserRepository) Update(ctx context.Context, user *entity.User) error {
 	user.UpdatedAt = time.Now()