feat: restructure routing and authentication for hidden layout and user login

frontend/src/routes/app/_hiddenLayout/add-product.tsx

@@ -0,0 +1,17 @@
+import { createFileRoute, redirect } from "@tanstack/react-router";
+import { isAuthenticated } from "../../../utils/auth";
+
+export const Route = createFileRoute("/app/_hiddenLayout/add-product")({
+  beforeLoad: async () => {
+    if (!(await isAuthenticated())) {
+      throw redirect({
+        to: "/login",
+      });
+    }
+  },
+  component: RouteComponent,
+});
+
+function RouteComponent() {
+  return <div>Hello "/app/add-product"!</div>;
+}