Fix auto-rename: add messages to useCallback deps

messages.length was stale in sendMessage callback because messages
was not in the dependency array.

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

frontend/src/context/ChatContext.tsx

@@ -175,7 +175,7 @@ export const ChatProvider: React.FC<{ children: ReactNode }> = ({ children }) =>
         setIsSending(false);
       }
     },
-    [currentConversation]
+    [currentConversation, messages]
   );
 
   const updateConversationTitle = useCallback(async (id: string, title: string) => {