fix: save session when diagram is modified via autosave

Add chartXML to dependency array of auto-save effect so that user
modifications in draw.io (captured via autosave) trigger session save.

components/chat-panel.tsx

@@ -639,6 +639,7 @@ export default function ChatPanel({
         sessionIsAvailable,
         currentSessionId,
         buildSessionData,
+        chartXML,
     ])
 
     // Update URL when a new session is created (first message sent)