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/css/builder.css

@@ -461,19 +461,21 @@
 
 /* ── Modal ───────────────────────────────────────────────────── */
 
-.modal-overlay[hidden] { display: none; }
-
 .modal-overlay {
+    display: none; /* hidden by default; JS adds .modal-open to show */
     position: fixed;
     inset: 0;
     background: rgba(0,0,0,0.45);
     z-index: 1000;
-    display: flex;
     align-items: center;
     justify-content: center;
     padding: 20px;
 }
 
+.modal-overlay.modal-open {
+    display: flex;
+}
+
 .modal-box {
     background: var(--bg-card);
     border-radius: 12px;

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;
     }