fix(document): fix test regressions from DocumentListItem migration

- Use documentService.getDocumentById() in detail_stillReturnsTrainingLabels
  so the Document.full entity graph eager-loads trainingLabels
- Flatten makeItem() factory in DocumentList.svelte.test.ts (nested
  document: {} overrides broke item.id / item.documentDate access)
- Remove { document: {} } wrapper from DocumentMultiSelect.svelte.spec.ts
  mock responses — component now reads body.items directly as flat items
- Flatten single nested item in page.svelte.test.ts document list test

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

backend/src/test/java/org/raddatz/familienarchiv/document/DocumentListItemIntegrationTest.java

@@ -90,8 +90,8 @@ class DocumentListItemIntegrationTest {
                 .trainingLabels(new HashSet<>(Set.of(TrainingLabel.KURRENT_RECOGNITION)))
                 .build());
 
-        // Document.full entity graph (used by findById) must still load trainingLabels
-        Document loaded = documentRepository.findById(saved.getId()).orElseThrow();
+        // Document.full entity graph (used by getDocumentById) must still load trainingLabels
+        Document loaded = documentService.getDocumentById(saved.getId());
 
         assertThat(loaded.getTrainingLabels()).containsExactly(TrainingLabel.KURRENT_RECOGNITION);
     }