fix: modal-overlay hidden attribute overridden by display:flex

The CSS display:flex on .modal-overlay was overriding the HTML hidden
attribute, making the modal always visible.

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

assets/styles/app.css

@@ -275,6 +275,10 @@ body {
     z-index: 300;
 }
 
+.modal-overlay[hidden] {
+    display: none;
+}
+
 .modal {
     background: white;
     border-radius: 12px;