feat(i18n): add reactive Alpine $t() magic and fix storefront language variable

- Register Alpine magic $t() for reactive translations in templates
- Dispatch i18n:ready event when translations load
- Fix base.html to use current_language instead of storefront_language

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/templates/storefront/base.html

@@ -377,7 +377,7 @@
         // Wrapped in DOMContentLoaded so deferred i18n.js has loaded
         document.addEventListener('DOMContentLoaded', async function() {
             const modules = {% block i18n_modules %}[]{% endblock %};
-            await I18n.init('{{ storefront_language | default("en") }}', modules);
+            await I18n.init('{{ current_language | default("en") }}', modules);
         });
     </script>