fix: match ChatResponse field names to API (user_message/assistant_message)

Frontend interface expected 'message' and 'response' but API returns
'user_message' and 'assistant_message'. Field name mismatch caused
undefined access and page crash.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/api/chat.ts

@@ -16,8 +16,8 @@ export interface ChatSession {
 
 export interface ChatResponse {
   session_id: string
-  message: ChatMessage
-  response: ChatMessage
+  user_message: ChatMessage
+  assistant_message: ChatMessage
 }
 
 export async function sendChatMessage(

frontend/src/components/chat/ChatPanel.tsx

@@ -66,7 +66,7 @@ export default function ChatPanel({ open, onClose, contextType, contextId }: Cha
       sendChatMessage(message, sessionId, contextType, contextId),
     onSuccess: (data) => {
       setSessionId(data.session_id)
-      setMessages((prev) => [...prev, data.message, data.response])
+      setMessages((prev) => [...prev, data.user_message, data.assistant_message])
       queryClient.invalidateQueries({ queryKey: ['chat-sessions'] })
     },
   })