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/pages/AgentChatPage.tsx

@@ -71,8 +71,8 @@ const AgentChatPage = () => {
           title: `与${agent.name}的对话`,
         })
 
-        if (response.data.success && response.data.data) {
-          setConversation(response.data.data)
+        if (response.success && response.data) {
+          setConversation(response.data)
           setMessages([])
         }
       } catch (err) {