fix: refine error messages for signup failure and company code

backend/internal/handler/auth_handler.go

@@ -460,7 +460,7 @@ func (h *AuthHandler) Signup(c *fiber.Ctx) error {
 		} else {
 			// If companyCode provided but not found, we should probably reject if we want strictness,
 			// or just treat as GENERAL user. Given the risk "존재하지 않는 테넌트도 저장됨", we should reject.
-			return errorJSON(c, fiber.StatusBadRequest, "Invalid company code.")
+			return errorJSON(c, fiber.StatusBadRequest, "해당하는 가족사(테넌트)를 찾을 수 없습니다.")
 		}
 	}