fix: 添加混合渠道警告确认框和过滤 prompt_cache_retention 参数

- 前端: EditAccountModal 和 CreateAccountModal 添加 409 mixed_channel_warning 处理
- 前端: 弹出确认框让用户确认混合渠道风险
- 后端: 过滤 OpenAI 请求中的 prompt_cache_retention 参数（上游不支持）
- 添加中英文翻译

Co-authored-by: factory-droid[bot] <138933559+factory-droid[bot]@users.noreply.github.com>

frontend/src/components/account/CreateAccountModal.vue

@@ -2157,6 +2157,46 @@ const handleClose = () => {
   emit('close')
 }
 
+// Helper function to create account with mixed channel warning handling
+const doCreateAccount = async (payload: any, confirmMixedChannelRisk = false) => {
+  if (confirmMixedChannelRisk) {
+    payload.confirm_mixed_channel_risk = true
+  }
+  
+  submitting.value = true
+  try {
+    await adminAPI.accounts.create(payload)
+    appStore.showSuccess(t('admin.accounts.accountCreated'))
+    emit('created')
+    handleClose()
+  } catch (error: any) {
+    // Handle 409 mixed_channel_warning - show confirmation dialog
+    if (error.response?.status === 409 && error.response?.data?.error === 'mixed_channel_warning') {
+      const details = error.response.data.details || {}
+      const groupName = details.group_name || 'Unknown'
+      const currentPlatform = details.current_platform || 'Unknown'
+      const otherPlatform = details.other_platform || 'Unknown'
+      
+      const confirmMessage = t('admin.accounts.mixedChannelWarning', {
+        groupName,
+        currentPlatform,
+        otherPlatform
+      })
+      
+      if (confirm(confirmMessage)) {
+        // Retry with confirmation flag
+        submitting.value = false
+        await doCreateAccount(payload, true)
+        return
+      }
+    } else {
+      appStore.showError(error.response?.data?.detail || t('admin.accounts.failedToCreate'))
+    }
+  } finally {
+    submitting.value = false
+  }
+}
+
 const handleSubmit = async () => {
   // For OAuth-based type, handle OAuth flow (goes to step 2)
   if (isOAuthFlow.value) {
@@ -2213,21 +2253,11 @@ const handleSubmit = async () => {
 
   form.credentials = credentials
 
-  submitting.value = true
-  try {
-    await adminAPI.accounts.create({
-      ...form,
-      group_ids: form.group_ids,
-      auto_pause_on_expired: autoPauseOnExpired.value
-    })
-    appStore.showSuccess(t('admin.accounts.accountCreated'))
-    emit('created')
-    handleClose()
-  } catch (error: any) {
-    appStore.showError(error.response?.data?.detail || t('admin.accounts.failedToCreate'))
-  } finally {
-    submitting.value = false
-  }
+  await doCreateAccount({
+    ...form,
+    group_ids: form.group_ids,
+    auto_pause_on_expired: autoPauseOnExpired.value
+  })
 }
 
 const goBackToBasicInfo = () => {