fix(review): address review blockers from PR #475

- CommentData.java: add @Nullable on annotationId to match codebase convention
- DashboardService: isEmpty() → isBlank() for commentPreview null-guard
- ChronikRow.svelte: always set aria-label on comment rows (not only when preview present)
- ChronikRow.svelte.spec.ts: add test for aria-label on comment row without preview

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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

@@ -149,7 +149,7 @@ public class DashboardService {
             UUID commentId = row.getCommentId();
             CommentData commentData = commentId != null ? commentDataByComment.get(commentId) : null;
             UUID annotationId = commentData != null ? commentData.annotationId() : null;
-            String commentPreview = commentData != null && !commentData.preview().isEmpty()
+            String commentPreview = commentData != null && !commentData.preview().isBlank()
                     ? commentData.preview() : null;
             return new ActivityFeedItemDTO(
                     org.raddatz.familienarchiv.audit.AuditKind.valueOf(row.getKind()),

backend/src/main/java/org/raddatz/familienarchiv/document/comment/CommentData.java

@@ -1,5 +1,6 @@
 package org.raddatz.familienarchiv.document.comment;
 
+import jakarta.annotation.Nullable;
 import java.util.UUID;
 
-public record CommentData(UUID annotationId, String preview) {}
+public record CommentData(@Nullable UUID annotationId, String preview) {}