feat(dashboard): add UploadSuccessBanner component

Transient post-upload banner for issue #296: singular/plural German copy,
aria-live=polite for screen readers, manual X dismiss, 8s auto-dismiss.
"Jetzt ergänzen →" CTA links directly to /enrich so seniors can continue
straight into the enrichment flow after a batch upload.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

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

@@ -0,0 +1,57 @@
+import { describe, it, expect, afterEach, vi } from 'vitest';
+import { cleanup, render } from 'vitest-browser-svelte';
+import { page } from 'vitest/browser';
+
+import UploadSuccessBanner from './UploadSuccessBanner.svelte';
+
+afterEach(() => {
+	cleanup();
+	vi.useRealTimers();
+});
+
+describe('UploadSuccessBanner', () => {
+	it('renders singular copy for count of 1', async () => {
+		render(UploadSuccessBanner, { count: 1, onClose: () => {} });
+		const status = page.getByRole('status');
+		await expect.element(status).toBeInTheDocument();
+		await expect.element(status).toHaveTextContent(/1 Dokument/);
+	});
+
+	it('renders plural copy for count greater than 1', async () => {
+		render(UploadSuccessBanner, { count: 3, onClose: () => {} });
+		await expect.element(page.getByRole('status')).toHaveTextContent(/3 Dokumente/);
+	});
+
+	it('exposes role=status with aria-live polite', async () => {
+		render(UploadSuccessBanner, { count: 1, onClose: () => {} });
+		await expect.element(page.getByRole('status')).toHaveAttribute('aria-live', 'polite');
+	});
+
+	it('renders a CTA link to /enrich', async () => {
+		render(UploadSuccessBanner, { count: 2, onClose: () => {} });
+		await expect
+			.element(page.getByRole('link', { name: /ergänzen/i }))
+			.toHaveAttribute('href', '/enrich');
+	});
+
+	it('invokes onClose when the close button is clicked', async () => {
+		const onClose = vi.fn();
+		render(UploadSuccessBanner, { count: 1, onClose });
+		const button = document.querySelector(
+			'[data-testid="upload-banner-close"]'
+		) as HTMLButtonElement | null;
+		expect(button).not.toBeNull();
+		button?.click();
+		expect(onClose).toHaveBeenCalledTimes(1);
+	});
+
+	it('auto-dismisses after 8000ms', async () => {
+		vi.useFakeTimers();
+		const onClose = vi.fn();
+		render(UploadSuccessBanner, { count: 1, onClose });
+		vi.advanceTimersByTime(7999);
+		expect(onClose).not.toHaveBeenCalled();
+		vi.advanceTimersByTime(2);
+		expect(onClose).toHaveBeenCalledTimes(1);
+	});
+});