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.svelte.spec.ts

@@ -57,4 +57,19 @@ describe('trapFocus action', () => {
 		// No trap after destroy → focus stays on the last button.
 		expect(document.activeElement).toBe(buttons[1]);
 	});
+
+	it('restores focus to the previously-focused element on destroy (WCAG 2.4.3)', () => {
+		const opener = document.createElement('button');
+		document.body.appendChild(opener);
+		nodes.push(opener);
+		opener.focus();
+		expect(document.activeElement).toBe(opener);
+
+		const { node } = makeContainer(['one', 'two']);
+		const handle = trapFocus(node);
+		expect(document.activeElement).not.toBe(opener);
+
+		handle.destroy();
+		expect(document.activeElement).toBe(opener);
+	});
 });

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?.();
 		}
 	};
 }