feat: voucher purchase flow

templates/voucher/preview_error.html.twig

@@ -0,0 +1,11 @@
+{# Shown inside the preview frame, outside the site layout #}
+<!DOCTYPE html>
+<html lang="de">
+<head>
+    <meta charset="UTF-8">
+    <title>Vorschau nicht verfügbar</title>
+</head>
+<body style="font-family: sans-serif; padding: 1rem;">
+    <p>{{ message }}</p>
+</body>
+</html>