feat(checkout): add useCheckoutPrefill hook for query-param prefill

Parses first_name/last_name/email/lock from the URL, validates email,
returns a stable { prefill, lock } for the checkout details form. 🤖

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: skvost

frontend/src/hooks/useCheckoutPrefill.ts

@@ -0,0 +1,48 @@
+import {useMemo} from "react";
+import {useSearchParams} from "react-router";
+
+export interface CheckoutPrefill {
+    first_name?: string;
+    last_name?: string;
+    email?: string;
+}
+
+export interface UseCheckoutPrefillResult {
+    prefill: CheckoutPrefill;
+    lock: boolean;
+}
+
+const EMAIL_REGEX = /^[^\s@]+@[^\s@]+\.[^\s@]+$/;
+
+export const useCheckoutPrefill = (): UseCheckoutPrefillResult => {
+    const [searchParams] = useSearchParams();
+
+    const firstNameParam = searchParams.get("first_name");
+    const lastNameParam = searchParams.get("last_name");
+    const emailParam = searchParams.get("email");
+    const lockParam = searchParams.get("lock");
+
+    return useMemo(() => {
+        const prefill: CheckoutPrefill = {};
+
+        const firstName = firstNameParam?.trim();
+        if (firstName) {
+            prefill.first_name = firstName;
+        }
+
+        const lastName = lastNameParam?.trim();
+        if (lastName) {
+            prefill.last_name = lastName;
+        }
+
+        const email = emailParam?.trim();
+        if (email && EMAIL_REGEX.test(email)) {
+            prefill.email = email;
+        }
+
+        const hasPrefill = Object.keys(prefill).length > 0;
+        const lock = hasPrefill && (lockParam === "true" || lockParam === "1");
+
+        return {prefill, lock};
+    }, [firstNameParam, lastNameParam, emailParam, lockParam]);
+};