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

frontend/src/routes/app/_hiddenLayout.tsx

@@ -0,0 +1,15 @@
+// routes/app/_layout.tsx (oder app.tsx als Parent)
+import { Outlet, createFileRoute } from "@tanstack/react-router";
+
+export const Route = createFileRoute("/app/_hiddenLayout")({
+  component: AppLayout,
+});
+
+function AppLayout() {
+  return (
+    <div>
+      <h1>Layout</h1>
+      <Outlet />
+    </div>
+  );
+}