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/routes/documents/page.svelte.test.ts

@@ -140,15 +140,12 @@ describe('documents/+ page', () => {
 				data: baseData({
 					items: [
 						{
-							document: {
-								id: 'd1',
-								title: 'Brief 1899',
-								status: 'TRANSCRIBED',
-								documentDate: '1899-04-14',
-								summary: '',
-								originalFilename: 'b1.pdf',
-								receivers: []
-							},
+							id: 'd1',
+							title: 'Brief 1899',
+							documentDate: '1899-04-14',
+							originalFilename: 'b1.pdf',
+							receivers: [],
+							tags: [],
 							matchData: {
 								titleOffsets: [],
 								senderMatched: false,