fix: resolve conversation API issues

1. Fixed double data unwrapping in AgentChatPage
2. Fixed API URL in sendMessageStream (use relative path /api/v1)

Verified working: AI can now respond to messages

frontend/src/api/conversationApi.ts

@@ -91,7 +91,8 @@ export const sendMessageStream = async (
   onError: (error: Error) => void
 ) => {
   try {
-    const response = await fetch(`${import.meta.env.VITE_API_BASE_URL}/conversations/message/stream`, {
+    // 使用相对路径，通过Vite代理转发到后端
+    const response = await fetch('/api/v1/conversations/message/stream', {
       method: 'POST',
       headers: {
         'Content-Type': 'application/json',