Add focus trapping for modal dialogs

- Focus moves to first focusable element when modal opens
- Tab/Shift+Tab cycles within modal only
- Escape key closes modal
- Focus returns to previously focused element on close

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

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

index.html

@@ -1142,12 +1142,64 @@
             }
         }
 
+        let previouslyFocusedElement = null;
+        let activeModalId = null;
+
         function openModal(id) {
-            document.getElementById(id).classList.add('active');
+            previouslyFocusedElement = document.activeElement;
+            activeModalId = id;
+            const modal = document.getElementById(id);
+            modal.classList.add('active');
+
+            // Focus first focusable element in modal
+            const focusable = modal.querySelectorAll('button, input, select, textarea, [tabindex]:not([tabindex="-1"])');
+            if (focusable.length > 0) {
+                setTimeout(() => focusable[0].focus(), 50);
+            }
+
+            // Add event listeners for focus trap and Escape
+            document.addEventListener('keydown', handleModalKeydown);
         }
 
         function closeModal(id) {
             document.getElementById(id).classList.remove('active');
+            document.removeEventListener('keydown', handleModalKeydown);
+            activeModalId = null;
+
+            // Restore focus to previously focused element
+            if (previouslyFocusedElement) {
+                previouslyFocusedElement.focus();
+                previouslyFocusedElement = null;
+            }
+        }
+
+        function handleModalKeydown(e) {
+            if (!activeModalId) return;
+            const modal = document.getElementById(activeModalId);
+
+            // Close on Escape
+            if (e.key === 'Escape') {
+                e.preventDefault();
+                closeModal(activeModalId);
+                return;
+            }
+
+            // Trap focus on Tab
+            if (e.key === 'Tab') {
+                const focusable = modal.querySelectorAll('button, input, select, textarea, [tabindex]:not([tabindex="-1"])');
+                if (focusable.length === 0) return;
+
+                const first = focusable[0];
+                const last = focusable[focusable.length - 1];
+
+                if (e.shiftKey && document.activeElement === first) {
+                    e.preventDefault();
+                    last.focus();
+                } else if (!e.shiftKey && document.activeElement === last) {
+                    e.preventDefault();
+                    first.focus();
+                }
+            }
         }
 
         async function verifyPin() {