Fix remaining merchant surfaces still using merchant.ladill.com/q URLs.

Storefront QR cards, slug preview, payment callbacks, and document downloads now use LadillLink so displayed and encoded links match ladl.link.

Co-authored-by: Cursor <cursoragent@cursor.com>

config/link.php

@@ -0,0 +1,5 @@
+<?php
+
+return [
+    'public_domain' => env('LINK_PUBLIC_DOMAIN', 'ladl.link'),
+];