feat: add save feedback prompts, featured agent fields, dynamic Dify API config, and chat improvements

- Add success/error feedback messages near submit buttons in all admin forms
- Display success prompt for 1s before redirect after save
- Show API error details on save failure
- Add isFeatured/featuredOrder fields to Agent model and admin UI
- Add difyApiUrl/difyApiKey fields for per-agent Dify API configuration
- Show featured badge column in agent admin list
- Display featured agents on homepage sorted by order
- Refactor chat page streaming with AbortController and stable userId
- Improve Dify API proxy to use per-agent credentials

测试/测试1.txt

@@ -1,14 +0,0 @@
-发送对话消息接口如下，该接口能正常调用
-curl -X POST 'http://df.clkeji.com/v1/chat-messages' \
---header 'Authorization: Bearer app-lbe2lglt7taGtZk0dG7pAhbx' \
---header 'Content-Type: application/json' \
---data '{
-  "inputs": {},
-  "query": "What are the specs of the iPhone 13 Pro Max?",
-  "response_mode": "streaming",
-  "conversation_id": "",
-  "user": "abc-123"
-}'
-
-问题描述：点击对话，功能异常，请排查原因并修复
-