feat(#145): add DashboardMentions widget component

Shows unread mention notifications as a dashboard widget. Renders
nothing when the mentions list is empty.

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

frontend/src/lib/components/DashboardMentions.svelte.spec.ts

@@ -0,0 +1,58 @@
+import { describe, it, expect, afterEach } from 'vitest';
+import { cleanup, render } from 'vitest-browser-svelte';
+import { page } from 'vitest/browser';
+
+import DashboardMentions from './DashboardMentions.svelte';
+
+afterEach(cleanup);
+
+type NotificationDTO = {
+	id: string;
+	type: 'REPLY' | 'MENTION';
+	documentId?: string;
+	read: boolean;
+	createdAt: string;
+	actorName?: string;
+};
+
+function makeMention(overrides: Partial<NotificationDTO> = {}): NotificationDTO {
+	return {
+		id: 'notif-1',
+		type: 'MENTION',
+		documentId: 'doc-abc',
+		read: false,
+		createdAt: '2026-01-15T10:00:00Z',
+		actorName: 'Anna Schmidt',
+		...overrides
+	};
+}
+
+describe('DashboardMentions', () => {
+	it('renders nothing when mentions list is empty', async () => {
+		render(DashboardMentions, { mentions: [] });
+		const widget = page.getByTestId('dashboard-mentions');
+		await expect.element(widget).not.toBeInTheDocument();
+	});
+
+	it('shows a heading when mentions are present', async () => {
+		render(DashboardMentions, { mentions: [makeMention()] });
+		const widget = page.getByTestId('dashboard-mentions');
+		await expect.element(widget).toBeInTheDocument();
+	});
+
+	it('renders one row per mention with link to document', async () => {
+		const mentions = [
+			makeMention({ id: 'n1', documentId: 'doc-1', actorName: 'Anna' }),
+			makeMention({ id: 'n2', documentId: 'doc-2', actorName: 'Bob' })
+		];
+		render(DashboardMentions, { mentions });
+		const links = page.getByRole('link');
+		await expect.element(links.nth(0)).toHaveAttribute('href', '/documents/doc-1');
+		await expect.element(links.nth(1)).toHaveAttribute('href', '/documents/doc-2');
+	});
+
+	it('shows actor name in each row', async () => {
+		render(DashboardMentions, { mentions: [makeMention({ actorName: 'Maria Müller' })] });
+		await expect.element(page.getByText('Maria Müller')).toBeInTheDocument();
+	});
+});