fix(shared): trapFocus restores focus to the opener on destroy (#692)

When the bottom sheet closes, focus returns to the element that was focused
before it opened instead of being dropped to document.body (WCAG 2.4.3,
Architect + UX review).

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

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

@@ -14,6 +14,9 @@ const FOCUSABLE_SELECTOR = [
 ].join(',');
 
 export function trapFocus(node: HTMLElement) {
+	// Remember what had focus so it can be restored when the overlay closes
+	// (WCAG 2.4.3 — don't strand keyboard/AT users at the top of the page).
+	const previouslyFocused = document.activeElement as HTMLElement | null;
 	const focusable = () => Array.from(node.querySelectorAll<HTMLElement>(FOCUSABLE_SELECTOR));
 
 	function onKeydown(event: KeyboardEvent) {
@@ -38,6 +41,7 @@ export function trapFocus(node: HTMLElement) {
 	return {
 		destroy() {
 			node.removeEventListener('keydown', onKeydown);
+			previouslyFocused?.focus?.();
 		}
 	};
 }