fix(document): add rel=noopener noreferrer to viewer download link (CWE-1022)

The error-state download link opened with target="_blank" but no rel,
exposing the opener to reverse tabnavbabbing. Add rel="noopener
noreferrer". Same-origin so low severity, but a one-token fix in a file
this issue already touches.

Refs #708

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

frontend/src/lib/document/DocumentViewer.svelte

@@ -72,6 +72,7 @@ let {
 				<a
 					href="/api/documents/{doc.id}/file"
 					target="_blank"
+					rel="noopener noreferrer"
 					class="text-sm underline hover:text-white"
 				>
 					{m.doc_download_link()}