fix(document): localize PdfViewer render-error message and download link

The error state showed a hardcoded German string ("Fehler beim Laden
der PDF" / "Direkt öffnen") to all users regardless of locale. Use the
localized doc_render_failed and doc_download_link messages so the
recovery path (message + working download link) is honest in de/en/es.

Refs #708

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/lib/document/viewer/PdfViewer.svelte.test.ts

@@ -3,9 +3,39 @@ import { cleanup, render } from 'vitest-browser-svelte';
 import { page } from 'vitest/browser';
 import PdfViewer from './PdfViewer.svelte';
 import { makeFakeLibLoader } from './testHelpers';
+import { m } from '$lib/paraglide/messages.js';
 
 afterEach(cleanup);
 
+function makeFailingLibLoader() {
+	const lib = {
+		GlobalWorkerOptions: { workerSrc: '' },
+		getDocument: vi.fn().mockReturnValue({
+			promise: Promise.reject(new Error('JBig2 failed to initialize'))
+		}),
+		TextLayer: class {
+			render() {
+				return Promise.resolve();
+			}
+			cancel() {}
+		}
+	} as unknown as typeof import('pdfjs-dist');
+	return vi.fn().mockResolvedValue([lib, { default: '' }] as const);
+}
+
+describe('PdfViewer — render failure', () => {
+	it('shows the localized failure message and a download link, not a blank canvas', async () => {
+		render(PdfViewer, {
+			url: '/api/documents/test/file',
+			documentId: 'test',
+			libLoader: makeFailingLibLoader()
+		});
+
+		await expect.element(page.getByText(m.doc_render_failed())).toBeVisible();
+		await expect.element(page.getByRole('link', { name: m.doc_download_link() })).toBeVisible();
+	});
+});
+
 describe('PdfViewer — empty / error states', () => {
 	it('renders the no-file placeholder when url is empty', async () => {
 		render(PdfViewer, { url: '', libLoader: makeFakeLibLoader() });