Fix admin route name mismatch in user_new and user_created templates

Templates referenced 'admin_user_new' (stegasoo convention) but the
soosef route is named 'admin_new_user'. Caused 500 error when clicking
"Add User" from admin panel.

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

frontends/web/templates/admin/user_new.html

@@ -131,7 +131,7 @@ form.addEventListener('submit', async function(e) {
     const formData = new FormData(form);
 
     try {
-        const response = await fetch('{{ url_for("admin_user_new") }}', {
+        const response = await fetch('{{ url_for("admin_new_user") }}', {
             method: 'POST',
             body: formData,
             headers: { 'X-Requested-With': 'XMLHttpRequest' }