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

@@ -171,14 +171,14 @@ function handleAnnotationClick(id: string) {
 	</div>
 {:else if renderer.error}
 	<div class="flex h-full w-full flex-col items-center justify-center gap-3 bg-pdf-bg text-ink-3">
-		<p class="font-sans text-sm text-red-400">Fehler beim Laden der PDF</p>
+		<p class="font-sans text-sm text-red-400">{m.doc_render_failed()}</p>
 		<a
 			href={url}
 			target="_blank"
 			rel="noopener noreferrer"
 			class="font-sans text-xs text-primary underline hover:text-ink-2"
 		>
-			Direkt öffnen
+			{m.doc_download_link()}
 		</a>
 	</div>
 {:else}