Persist expandedFolders in local storage

src/App.tsx

@@ -160,6 +160,13 @@ export default function App() {
     localStorage.setItem("textdb.lineNumbers", String(showLineNumbers));
   }, [showLineNumbers]);
 
+  useEffect(() => {
+    localStorage.setItem(
+      "textdb.expandedFolders",
+      JSON.stringify(Array.from(expandedFolders))
+    );
+  }, [expandedFolders]);
+
   useEffect(() => {
     if (selectedTextId) {
       localStorage.setItem("textdb.selectedTextId", selectedTextId);