Fix visibility check for elements in fixed-position modals

offsetParent returns null for elements inside position:fixed containers.
Use getComputedStyle instead to check display and visibility.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

index.html

@@ -1186,7 +1186,9 @@
         function getFocusableElements(container) {
             const elements = container.querySelectorAll('button, input:not([type="hidden"]), select, textarea, [tabindex]:not([tabindex="-1"])');
             return Array.from(elements).filter(el => {
-                return el.offsetParent !== null && !el.disabled;
+                if (el.disabled) return false;
+                const style = window.getComputedStyle(el);
+                return style.display !== 'none' && style.visibility !== 'hidden';
             });
         }