fix: update dropbox-sync API to merge non-editable fields, fix setCors crash and update fallback asset

src/App.tsx

@@ -295,7 +295,7 @@ export default function App() {
           const syncRes = await fetch('/api/dropbox-sync', {
             method: 'POST',
             headers: { 'Content-Type': 'application/json' },
-            body: JSON.stringify({ rows, fileMeta })
+            body: JSON.stringify({ rows, headers, fileMeta })
           });
           
           if (syncRes.ok) {