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>

frontend/src/lib/document/DocumentMultiSelect.svelte.spec.ts

@@ -22,7 +22,7 @@ function mockSearchResponse(items: ReturnType<typeof docFactory>[]) {
 		'fetch',
 		vi.fn().mockResolvedValue({
 			ok: true,
-			json: vi.fn().mockResolvedValue({ items: items.map((document) => ({ document })) })
+			json: vi.fn().mockResolvedValue({ items })
 		})
 	);
 }
@@ -91,10 +91,7 @@ describe('DocumentMultiSelect — search and select', () => {
 		const fetchMock = vi.fn().mockResolvedValue({
 			ok: true,
 			json: vi.fn().mockResolvedValue({
-				items: [
-					{ document: docFactory('d1', 'Already attached') },
-					{ document: docFactory('d2', 'Not attached') }
-				]
+				items: [docFactory('d1', 'Already attached'), docFactory('d2', 'Not attached')]
 			})
 		});
 		vi.stubGlobal('fetch', fetchMock);