Persist showLineNumbers state in localStorage

src/App.tsx

@@ -121,6 +121,10 @@ export default function App() {
     localStorage.setItem("textdb.sidebarCollapsed", String(sidebarCollapsed));
   }, [sidebarCollapsed]);
 
+  useEffect(() => {
+    localStorage.setItem("textdb.lineNumbers", String(showLineNumbers));
+  }, [showLineNumbers]);
+
   useEffect(() => {
     if (selectedTextId) {
       localStorage.setItem("textdb.selectedTextId", selectedTextId);
@@ -156,8 +160,7 @@ export default function App() {
   const historyIconSrc = theme === "light" ? historyIconBright : historyIcon;
 
   const lineNumbers = useMemo(() => {
-    const count = Math.max(body.split("
-").length, 1);
+    const count = Math.max(body.split("\n").length, 1);
     return Array.from({ length: count }, (_, index) => index + 1);
   }, [body]);