fix(frontend): localize agent creation errors

frontend/app/javascript/dashboard/i18n/locale/en/agentMgmt.json

@@ -46,6 +46,7 @@
       "API": {
         "SUCCESS_MESSAGE": "Agent added successfully",
         "EXIST_MESSAGE": "Agent email already in use, Please try another email address",
+        "LIMIT_MESSAGE": "Agent limit reached. Please upgrade your plan or add more seats.",
         "ERROR_MESSAGE": "Could not connect to Woot Server, Please try again later"
       }
     },

frontend/app/javascript/dashboard/i18n/locale/zh_CN/agentMgmt.json

@@ -46,6 +46,7 @@
       "API": {
         "SUCCESS_MESSAGE": "添加客服成功",
         "EXIST_MESSAGE": "该邮箱已被注册，请输入新的电子邮箱",
+        "LIMIT_MESSAGE": "客服数量已达到上限，请升级套餐或增加客服名额",
         "ERROR_MESSAGE": "无法连接服务器，请稍后再试"
       }
     },

frontend/app/javascript/dashboard/routes/dashboard/settings/agents/AddAgent.vue

@@ -90,23 +90,19 @@ const addAgent = async () => {
       emit('close');
     }
   } catch (error) {
-    const {
-      response: {
-        data: {
-          error: errorResponse = '',
-          attributes: attributes = [],
-          message: attrError = '',
-        } = {},
-      } = {},
-    } = error;
+    const status = error?.response?.status;
+    const data = error?.response?.data;
+    const attributes = data?.attributes || [];
 
-    let errorMessage = '';
-    if (error?.response?.status === 422 && !attributes.includes('base')) {
-      errorMessage = t('AGENT_MGMT.ADD.API.EXIST_MESSAGE');
+    if (status === 422 && !attributes.includes('base')) {
+      useAlert(t('AGENT_MGMT.ADD.API.EXIST_MESSAGE'));
+    } else if (status === 402 || data?.error?.code === 'PAYMENT_REQUIRED') {
+      useAlert(t('AGENT_MGMT.ADD.API.LIMIT_MESSAGE'));
     } else {
-      errorMessage = t('AGENT_MGMT.ADD.API.ERROR_MESSAGE');
+      // Never pass the structured API error object to the toast. It renders as
+      // raw JSON and the backend message is not localized.
+      useAlert(t('AGENT_MGMT.ADD.API.ERROR_MESSAGE'));
     }
-    useAlert(errorResponse || attrError || errorMessage);
   }
 };