Fix modal auto-opening on builder page load

CSS display:flex on .modal-overlay overrides the HTML hidden attribute.
Add [hidden] selector to restore display:none when the attribute is present,
so the modal is correctly hidden on load and closeable via Cancel.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

assets/css/builder.css

@@ -461,6 +461,8 @@
 
 /* ── Modal ───────────────────────────────────────────────────── */
 
+.modal-overlay[hidden] { display: none; }
+
 .modal-overlay {
     position: fixed;
     inset: 0;