feat(shared): add trapFocus action for modal overlays (#692)

Focuses the first focusable on mount and wraps Tab/Shift+Tab within the node.
Used by the Stammbaum mobile bottom sheet (NFR-A11Y-004).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/lib/shared/actions/trapFocus.ts

@@ -0,0 +1,43 @@
+/**
+ * Trap keyboard focus within a node and move focus to its first focusable
+ * element on mount. Used by modal-style overlays such as the Stammbaum mobile
+ * bottom sheet (#692, NFR-A11Y-004). Tab from the last focusable wraps to the
+ * first and Shift+Tab from the first wraps to the last.
+ */
+const FOCUSABLE_SELECTOR = [
+	'a[href]',
+	'button:not([disabled])',
+	'input:not([disabled])',
+	'select:not([disabled])',
+	'textarea:not([disabled])',
+	'[tabindex]:not([tabindex="-1"])'
+].join(',');
+
+export function trapFocus(node: HTMLElement) {
+	const focusable = () => Array.from(node.querySelectorAll<HTMLElement>(FOCUSABLE_SELECTOR));
+
+	function onKeydown(event: KeyboardEvent) {
+		if (event.key !== 'Tab') return;
+		const items = focusable();
+		if (items.length === 0) return;
+		const first = items[0];
+		const last = items[items.length - 1];
+		const active = document.activeElement;
+		if (event.shiftKey && active === first) {
+			event.preventDefault();
+			last.focus();
+		} else if (!event.shiftKey && active === last) {
+			event.preventDefault();
+			first.focus();
+		}
+	}
+
+	node.addEventListener('keydown', onKeydown);
+	focusable()[0]?.focus();
+
+	return {
+		destroy() {
+			node.removeEventListener('keydown', onKeydown);
+		}
+	};
+}