test(shared): cover BackButton and OverflowPillButton

BackButton: visible vs aria-only label branches, custom class
application, history.back() click handler.

OverflowPillButton: +N pill render, aria-expanded matrix
(closed default → open after click), per-person link rendering with
correct href, Escape closes the dropdown.

Both are reused widely; their coverage closes the line and function gap
left after the DocumentTopBar split inflated the denominator.

Refs #496.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/lib/shared/primitives/BackButton.svelte.test.ts

@@ -0,0 +1,50 @@
+import { describe, it, expect, vi, afterEach } from 'vitest';
+import { cleanup, render } from 'vitest-browser-svelte';
+import { page } from 'vitest/browser';
+import BackButton from './BackButton.svelte';
+
+afterEach(cleanup);
+
+describe('BackButton', () => {
+	it('renders the visible label by default (showLabel=true)', async () => {
+		render(BackButton, { props: {} });
+
+		await expect.element(page.getByRole('button', { name: /^zurück$/i })).toBeVisible();
+	});
+
+	it('hides the visible label when showLabel is false', async () => {
+		render(BackButton, { props: { showLabel: false } });
+
+		const btn = (await page.getByRole('button').element()) as HTMLButtonElement;
+		// The label is exposed via aria-label only when showLabel=false.
+		expect(btn.getAttribute('aria-label')).toBe('Zurück');
+		expect(btn.textContent?.trim()).toBe('');
+	});
+
+	it('does not set an aria-label when the visible label is shown', async () => {
+		render(BackButton, { props: { showLabel: true } });
+
+		const btn = (await page.getByRole('button').element()) as HTMLButtonElement;
+		expect(btn.getAttribute('aria-label')).toBeNull();
+	});
+
+	it('applies the supplied class string to the button', async () => {
+		render(BackButton, { props: { class: 'custom-class' } });
+
+		const btn = (await page.getByRole('button').element()) as HTMLButtonElement;
+		expect(btn.classList.contains('custom-class')).toBe(true);
+	});
+
+	it('calls history.back() when clicked', async () => {
+		const backSpy = vi.spyOn(globalThis.history, 'back').mockImplementation(() => {});
+		try {
+			render(BackButton, { props: {} });
+
+			await page.getByRole('button').click();
+
+			expect(backSpy).toHaveBeenCalledOnce();
+		} finally {
+			backSpy.mockRestore();
+		}
+	});
+});