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.

app/[lang]/page.tsx

@@ -179,6 +179,7 @@ export default function Home() {
                                             saveAndExit: false,
                                             noSaveBtn: true,
                                             noExitBtn: true,
+                                            modified: false,
                                             dark: darkMode,
                                             lang: currentLang,
                                         }}

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)