feat(frontend): finished login form with validation

frontend/src/routes/_admin.tsx

@@ -0,0 +1,13 @@
+import {createFileRoute, redirect} from "@tanstack/react-router";
+
+// src/routes/_authenticated.tsx
+export const Route = createFileRoute("/_admin")({
+    beforeLoad: async () => {
+        const auth = localStorage.getItem("auth_token");
+        if (!auth) {
+            throw redirect({
+                to: "/login",
+            });
+        }
+    },
+});