feat: handle redirects for admin in /order page and implement next query param on login

shop/components/login-form.tsx

@@ -31,12 +31,14 @@ export function LoginForm({
     setIsLoading(true);
     setError(null);
 
+    const nextParam = typeof window !== 'undefined' ? new URLSearchParams(window.location.search).get('next') : null;
+
     try {
       const res = await signIn(email, password);
       if (res.role === "admin") {
         router.push("/admin/einstellungen");
       } else {
-        router.push("/my-customers");
+        router.push(nextParam || "/my-customers");
       }
     } catch (error: unknown) {
       setError(error instanceof Error ? error.message : "An error occurred");