fix(csrf): send X-XSRF-TOKEN on all client-side mutating fetch calls

hooks.server.ts already forwards the CSRF token for server-side fetch
(form actions, load). Client-side XHR calls bypassed it, causing Spring
Security to return 403 before PermissionAspect even ran.

Adds getCsrfToken/withCsrf/makeCsrfFetch to cookies.ts.
useTranscriptionBlocks wraps its injectable fetchImpl with makeCsrfFetch
(covers all block mutations and saveBlockWithConflictRetry).
useBlockAutoSave, TranscriptionEditView, BulkDocumentEditLayout,
OcrTrainingCard, and SegmentationTrainingCard apply withCsrf inline.

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

frontend/src/lib/document/BulkDocumentEditLayout.svelte

@@ -17,6 +17,7 @@ import PdfViewer from '$lib/document/viewer/PdfViewer.svelte';
 import { bulkTitleFromFilename } from '$lib/document/filename';
 import type { Tag } from '$lib/tag/TagInput.svelte';
 import type { components } from '$lib/generated/api';
+import { withCsrf } from '$lib/shared/cookies';
 
 type Person = components['schemas']['Person'];
 
@@ -183,7 +184,10 @@ async function saveUpload() {
 		// FormData with per-chunk progress. Session cookie is sent automatically
 		// by the browser for same-origin requests.
 		try {
-			const res = await fetch('/api/documents/quick-upload', { method: 'POST', body: formData });
+			const res = await fetch(
+				'/api/documents/quick-upload',
+				withCsrf({ method: 'POST', body: formData })
+			);
 			const body = await res.json().catch(() => ({ errors: [] }));
 			const errorFilenames = new Set<string>(
 				(body.errors ?? []).map((err: { filename: string }) => err.filename)