feat(web): 素材凭据输入框增加已配置占位符

编辑 54/61 渠道时 AccessKey/SecretKey 输入框此前完全留空，容易误以为
凭据未保存。已配置时占位符显示"已配置，留空保持不变"，未配置时显示
"请输入 AccessKey/SecretKey"。后端仍不回传真实密钥，仅依据
asset_credential_configured 状态区分占位符文案。顺带补齐凭据卡片
已有文案的英文翻译。

web/src/components/table/channels/modals/EditChannelModal.jsx

@@ -2566,12 +2566,22 @@ const EditChannelModal = (props) => {
                           label='AccessKey'
                           mode='password'
                           autoComplete='new-password'
+                          placeholder={
+                            inputs.asset_credential_configured
+                              ? t('已配置，留空保持不变')
+                              : t('请输入 AccessKey')
+                          }
                         />
                         <Form.Input
                           field='asset_credential.secret_key'
                           label='SecretKey'
                           mode='password'
                           autoComplete='new-password'
+                          placeholder={
+                            inputs.asset_credential_configured
+                              ? t('已配置，留空保持不变')
+                              : t('请输入 SecretKey')
+                          }
                         />
                         {inputs.type === 61 ? (
                           <Form.Input

web/src/i18n/locales/en.json

@@ -2723,6 +2723,16 @@
     "豆包视频（素材网关）": "Doubao Video (Asset Gateway)",
     "素材 API 地址": "Asset API Base URL",
     "素材库接口调用该地址下的 /openApi/portrait，为素材网关专属；留空则使用渠道 API 地址。火山官方地址不提供此接口。": "Asset APIs call /openApi/portrait under this address (asset gateway only); leave empty to use the channel API address. The official Volcengine address does not provide this API.",
+    "已配置，留空保持不变": "Configured; leave empty to keep the current value",
+    "请输入 AccessKey": "Enter the AccessKey",
+    "请输入 SecretKey": "Enter the SecretKey",
+    "已配置素材凭证；留空不会覆盖现有凭证。": "Asset credentials configured; leaving the fields empty will not overwrite them.",
+    "未配置素材凭证；素材上传接口将不可用。": "No asset credentials configured; asset upload APIs will be unavailable.",
+    "移动云素材凭证": "China Mobile Cloud Asset Credentials",
+    "火山素材凭证": "Volcengine Asset Credentials",
+    "项目编码 ProjectCode": "Project Code (ProjectCode)",
+    "火山项目编码，可选": "Volcengine project code, optional",
+    "请同时填写素材 AccessKey 和 SecretKey": "Fill in both asset AccessKey and SecretKey",
     "账单": "Bills",
     "账户充值": "Account recharge",
     "账户已删除！": "Account has been deleted!",