fix(frontend): 修复运维设置对话框保存按钮始终禁用的问题

后端默认 alert.enabled=true 但 recipients 为空，前端验证将其视为
错误并阻断保存按钮。移除该阻断性验证，改为保存时自动禁用无收件人
的邮件通知配置。

frontend/src/views/admin/ops/components/OpsSettingsDialog.vue

@@ -131,15 +131,7 @@ const validation = computed(() => {
     }
   }
 
-  // 验证邮件配置
-  if (emailConfig.value) {
-    if (emailConfig.value.alert.enabled && emailConfig.value.alert.recipients.length === 0) {
-      errors.push(t('admin.ops.email.validation.alertRecipientsRequired'))
-    }
-    if (emailConfig.value.report.enabled && emailConfig.value.report.recipients.length === 0) {
-      errors.push(t('admin.ops.email.validation.reportRecipientsRequired'))
-    }
-  }
+  // 邮件配置: 启用但无收件人时不阻断保存, 保存时会自动禁用
 
   // 验证高级设置
   if (advancedSettings.value) {
@@ -181,6 +173,15 @@ async function saveAllSettings() {
 
   saving.value = true
   try {
+    // 无收件人时自动禁用邮件通知
+    if (emailConfig.value) {
+      if (emailConfig.value.alert.enabled && emailConfig.value.alert.recipients.length === 0) {
+        emailConfig.value.alert.enabled = false
+      }
+      if (emailConfig.value.report.enabled && emailConfig.value.report.recipients.length === 0) {
+        emailConfig.value.report.enabled = false
+      }
+    }
     await Promise.all([
       runtimeSettings.value ? opsAPI.updateAlertRuntimeSettings(runtimeSettings.value) : Promise.resolve(),
       emailConfig.value ? opsAPI.updateEmailNotificationConfig(emailConfig.value) : Promise.resolve(),