Fix modal visibility using CSS class instead of HTML hidden attribute

Replace hidden attribute + [hidden] CSS selector with a .modal-open
class toggle. The hidden attribute approach was unreliable because
CSS display:flex was winning the specificity battle. Class-based
show/hide is explicit and not subject to attribute/cascade conflicts.

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

assets/js/builder.js

@@ -257,12 +257,12 @@
 
         updateModalFields();
         updateAttrRadioStyles();
-        modal.hidden = false;
+        modal.classList.add('modal-open');
         document.getElementById('modal-name').focus();
     }
 
     function closeModal() {
-        document.getElementById('prayer-modal').hidden = true;
+        document.getElementById('prayer-modal').classList.remove('modal-open');
         editingPrayerId = null;
     }