feat: Add document download link component, refactor download URL resolution with expiration checks

frontend/src/viewer/DocumentViewerLayout.tsx

@@ -2,6 +2,7 @@ import { useCallback, useMemo, useRef } from 'react';
 import type { JSX } from 'react';
 import DocumentInfoPanel from './components/DocumentInfoPanel';
 import UnifiedDocumentViewer from './UnifiedDocumentViewer';
+import { resolveDocumentDownloadHref } from '../documents/documentActions';
 
 import type { Document } from '../types/documents';
 
@@ -56,13 +57,13 @@ const DocumentViewerLayout = ({
 
   const renderViewportPane = useCallback(() => (
     <div className="document-viewer__viewport" ref={viewportRef}>
-      {!document?.current_version?.download?.url ? (
+      {!resolveDocumentDownloadHref(document) ? (
         <div className="document-viewer__message">{previewLoadingMessage}</div>
       ) : (
         previewContent
       )}
     </div>
-  ), [previewContent, document?.current_version?.download?.url, previewLoadingMessage, viewportRef]);
+  ), [previewContent, document, previewLoadingMessage, viewportRef]);
 
   const viewportPane = renderViewportPane();