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.svelte.spec.ts

@@ -0,0 +1,60 @@
+import { describe, it, expect, afterEach } from 'vitest';
+
+const { trapFocus } = await import('./trapFocus');
+
+describe('trapFocus action', () => {
+	const nodes: HTMLElement[] = [];
+
+	function makeContainer(buttonLabels: string[]): {
+		node: HTMLElement;
+		buttons: HTMLButtonElement[];
+	} {
+		const node = document.createElement('div');
+		const buttons = buttonLabels.map((label) => {
+			const b = document.createElement('button');
+			b.textContent = label;
+			node.appendChild(b);
+			return b;
+		});
+		document.body.appendChild(node);
+		nodes.push(node);
+		return { node, buttons };
+	}
+
+	afterEach(() => {
+		nodes.forEach((n) => n.remove());
+		nodes.length = 0;
+	});
+
+	it('focuses the first focusable element on mount', () => {
+		const { node, buttons } = makeContainer(['one', 'two']);
+		trapFocus(node);
+		expect(document.activeElement).toBe(buttons[0]);
+	});
+
+	it('wraps Tab from the last focusable back to the first', () => {
+		const { node, buttons } = makeContainer(['one', 'two']);
+		trapFocus(node);
+		buttons[1].focus();
+		node.dispatchEvent(new KeyboardEvent('keydown', { key: 'Tab', bubbles: true }));
+		expect(document.activeElement).toBe(buttons[0]);
+	});
+
+	it('wraps Shift+Tab from the first focusable to the last', () => {
+		const { node, buttons } = makeContainer(['one', 'two']);
+		trapFocus(node);
+		buttons[0].focus();
+		node.dispatchEvent(new KeyboardEvent('keydown', { key: 'Tab', shiftKey: true, bubbles: true }));
+		expect(document.activeElement).toBe(buttons[1]);
+	});
+
+	it('removes its listener on destroy', () => {
+		const { node, buttons } = makeContainer(['one', 'two']);
+		const handle = trapFocus(node);
+		handle.destroy();
+		buttons[1].focus();
+		node.dispatchEvent(new KeyboardEvent('keydown', { key: 'Tab', bubbles: true }));
+		// No trap after destroy → focus stays on the last button.
+		expect(document.activeElement).toBe(buttons[1]);
+	});
+});

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

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+/**
+ * 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);
+		}
+	};
+}