feat(#145): add DashboardResumeStrip component

- Component reads familienarchiv.lastVisited from localStorage and
  shows a 'Zuletzt geöffnet' link to the last-visited document
- Renders nothing when no localStorage entry exists
- Document detail page writes id+title to localStorage on mount

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

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

@@ -0,0 +1,43 @@
+import { describe, it, expect, afterEach } from 'vitest';
+import { cleanup, render } from 'vitest-browser-svelte';
+import { page } from 'vitest/browser';
+
+import DashboardResumeStrip from './DashboardResumeStrip.svelte';
+
+afterEach(() => {
+	cleanup();
+	localStorage.clear();
+});
+
+describe('DashboardResumeStrip', () => {
+	it('renders nothing when no last-visited document in localStorage', async () => {
+		render(DashboardResumeStrip, {});
+		const strip = page.getByTestId('resume-strip');
+		await expect.element(strip).not.toBeInTheDocument();
+	});
+
+	it('shows the strip with link when localStorage has a document', async () => {
+		localStorage.setItem(
+			'familienarchiv.lastVisited',
+			JSON.stringify({ id: 'doc-123', title: 'Geburtsurkunde 1920' })
+		);
+		render(DashboardResumeStrip, {});
+		const strip = page.getByTestId('resume-strip');
+		await expect.element(strip).toBeInTheDocument();
+		const link = page.getByRole('link', { name: /Geburtsurkunde 1920/ });
+		await expect.element(link).toBeInTheDocument();
+		await expect.element(link).toHaveAttribute('href', '/documents/doc-123');
+	});
+
+	it('uses title fallback text when title is empty', async () => {
+		localStorage.setItem(
+			'familienarchiv.lastVisited',
+			JSON.stringify({ id: 'doc-456', title: '' })
+		);
+		render(DashboardResumeStrip, {});
+		const strip = page.getByTestId('resume-strip');
+		await expect.element(strip).toBeInTheDocument();
+		const link = page.getByRole('link');
+		await expect.element(link).toHaveAttribute('href', '/documents/doc-456');
+	});
+});