fix: enable autosave to sync user modifications in draw.io

Previously, user modifications made directly in draw.io (moving nodes,
editing text, etc.) were not captured by React state until an explicit
export was triggered.

This adds the autosave feature from react-drawio:
- Enable autosave={true} on DrawIoEmbed component
- Add onAutoSave callback to update chartXML on every change
- Keep React state in sync with draw.io editor state

app/[lang]/page.tsx

@@ -16,8 +16,13 @@ const drawioBaseUrl =
     process.env.NEXT_PUBLIC_DRAWIO_BASE_URL || "https://embed.diagrams.net"
 
 export default function Home() {
-    const { drawioRef, handleDiagramExport, onDrawioLoad, resetDrawioReady } =
-        useDiagram()
+    const {
+        drawioRef,
+        handleDiagramExport,
+        handleAutoSave,
+        onDrawioLoad,
+        resetDrawioReady,
+    } = useDiagram()
     const router = useRouter()
     const pathname = usePathname()
     // Extract current language from pathname (e.g., "/zh/about" → "zh")
@@ -164,6 +169,8 @@ export default function Home() {
                                         ref={drawioRef}
                                         onExport={handleDiagramExport}
                                         onLoad={handleDrawioLoad}
+                                        onAutoSave={handleAutoSave}
+                                        autosave={true}
                                         baseUrl={drawioBaseUrl}
                                         urlParameters={{
                                             ui: drawioUi,