fix(search): use in-memory sort for SENDER to include documents with null sender

INNER JOIN from Sort.by("sender.lastName") was excluding docs without a sender.

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

backend/src/main/java/org/raddatz/familienarchiv/service/DocumentService.java

@@ -291,11 +291,15 @@ public class DocumentService {
                 .and(hasTagPartial(tagQ))
                 .and(hasStatus(status));
 
-        Sort springSort = resolveSort(sort, dir);
         if (sort == DocumentSort.RECEIVER) {
             List<Document> results = documentRepository.findAll(spec);
             return sortByFirstReceiver(results, dir);
         }
+        if (sort == DocumentSort.SENDER) {
+            List<Document> results = documentRepository.findAll(spec);
+            return sortBySender(results, dir);
+        }
+        Sort springSort = resolveSort(sort, dir);
         return documentRepository.findAll(spec, springSort);
     }
 
@@ -312,6 +316,22 @@ public class DocumentService {
         };
     }
 
+    private List<Document> sortBySender(List<Document> documents, String dir) {
+        boolean ascending = "ASC".equalsIgnoreCase(dir);
+        Comparator<String> nullSafeComparator = (a, b) -> {
+            if (a.isEmpty() && b.isEmpty()) return 0;
+            if (a.isEmpty()) return ascending ? 1 : -1;
+            if (b.isEmpty()) return ascending ? -1 : 1;
+            return ascending ? a.compareTo(b) : b.compareTo(a);
+        };
+        return documents.stream()
+                .sorted(Comparator.comparing(doc -> {
+                    Person s = doc.getSender();
+                    return s != null ? s.getLastName() + " " + s.getFirstName() : "";
+                }, nullSafeComparator))
+                .toList();
+    }
+
     private List<Document> sortByFirstReceiver(List<Document> documents, String dir) {
         boolean ascending = "ASC".equalsIgnoreCase(dir);
         Comparator<String> nullSafeComparator = (a, b) -> {