✨ chore: Improve channel creation UX: defer "Fetch Model List" action until after creation

Previously, the "Fetch Model List" button was visible in the channel-creation view even though
it only functions once a channel record exists, leading to user confusion.

Changes introduced:
• Render the "Fetch Model List" button only when editing an existing channel (`isEdit === true`).
• Display an informational Banner in creation mode to remind users that the upstream model list
  can be fetched after the channel has been created.
• Refactored JSX to apply the above conditional rendering without altering existing logic.

This update streamlines the creation workflow and sets clearer expectations for users.

web/src/i18n/locales/en.json

@@ -1748,5 +1748,6 @@
   "快捷设置": "Quick settings",
   "批量创建时会在名称后自动添加随机后缀": "When creating in batches, a random suffix will be automatically added to the name",
   "额度必须大于0": "Quota must be greater than 0",
-  "生成数量必须大于0": "Generation quantity must be greater than 0"
+  "生成数量必须大于0": "Generation quantity must be greater than 0",
+  "创建后可在编辑渠道时获取上游模型列表": "After creation, you can get the upstream model list when editing the channel"
 }