fix(search): treat null sender.lastName as empty in sort key

A sender with lastName=null produced sort key "null Bob" which sorted
before names starting with lowercase letters (n < s, t, u, v...).
Now returns "" for null lastName, which the comparator places at end.

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

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

@@ -32,6 +32,7 @@ import java.util.HashMap;
 import java.util.HashSet;
 import java.util.List;
 import java.util.Map;
+import java.util.Objects;
 import java.util.Optional;
 import java.util.Set;
 import java.util.UUID;
@@ -327,7 +328,8 @@ public class DocumentService {
         return documents.stream()
                 .sorted(Comparator.comparing(doc -> {
                     Person s = doc.getSender();
-                    return s != null ? s.getLastName() + " " + s.getFirstName() : "";
+                    if (s == null || s.getLastName() == null) return "";
+                    return s.getLastName() + " " + Objects.toString(s.getFirstName(), "");
                 }, nullSafeComparator))
                 .toList();
     }

backend/src/test/java/org/raddatz/familienarchiv/service/DocumentServiceTest.java

@@ -1293,4 +1293,27 @@ class DocumentServiceTest {
         assertThat(result).hasSize(2);
         assertThat(result).extracting(Document::getTitle).containsExactly("Has Sender", "No Sender");
     }
+
+    @Test
+    void searchDocuments_senderSort_nullLastNameSortsToEnd() {
+        // Without fix: null lastName produces sort key "null Smith" which compares
+        // as 'n' (110) < 's' (115) and sorts BEFORE "smith" — wrong.
+        // With fix (Objects.toString → ""): key " Smith" sorts before real names but
+        // the sender-null-branch treats it as empty and places it at the end.
+        Person withRealName = Person.builder().id(UUID.randomUUID()).firstName("Alice").lastName("smith").build();
+        Person withNullLastName = Person.builder().id(UUID.randomUUID()).firstName("Bob").lastName(null).build();
+
+        Document docSmith = Document.builder().id(UUID.randomUUID()).title("smith doc").sender(withRealName).build();
+        Document docNullName = Document.builder().id(UUID.randomUUID()).title("Null lastname doc").sender(withNullLastName).build();
+
+        when(documentRepository.findAll(any(org.springframework.data.jpa.domain.Specification.class)))
+                .thenReturn(List.of(docNullName, docSmith));
+
+        List<Document> result = documentService.searchDocuments(
+                null, null, null, null, null, null, null, null, DocumentSort.SENDER, "asc");
+
+        // null lastName should sort to end (treated as empty), not before "smith" (as "null")
+        assertThat(result).extracting(Document::getTitle)
+                .containsExactly("smith doc", "Null lastname doc");
+    }
 }