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(