feat(dashboard): populate resume thumbnailUrl from Document

DashboardService now reads the URL from the Document's computed getter
instead of passing null, so the resume strip can display the real
thumbnail of whatever the user was last working on (#309).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/src/main/java/org/raddatz/familienarchiv/dashboard/DashboardService.java

@@ -82,7 +82,7 @@ public class DashboardService {
                 .toList();
 
         return new DashboardResumeDTO(docId, doc.getTitle(), caption, excerpt,
-                totalBlocks, pct, null, collaborators);
+                totalBlocks, pct, doc.getThumbnailUrl(), collaborators);
     }
 
     public DashboardPulseDTO getPulse(UUID userId) {