test(better auth): implementation start of better auth

frontend/src/routes/_adminLayout/admin/user/$userId.tsx

@@ -0,0 +1,9 @@
+import { createFileRoute } from '@tanstack/react-router'
+
+export const Route = createFileRoute('/_adminLayout/admin/user/$userId')({
+  component: RouteComponent,
+})
+
+function RouteComponent() {
+  return <div>Hello "/_adminLayout/admin/$userId"!</div>
+}