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)

frontend/src/lib/document/transcription/TranscriptionEditView.svelte

@@ -6,6 +6,7 @@ import TranscribeCoachEmptyState from '$lib/shared/help/TranscribeCoachEmptyStat
 import type { PersonMention, TranscriptionBlockData } from '$lib/shared/types';
 import { createBlockAutoSave } from '$lib/document/transcription/useBlockAutoSave.svelte';
 import { createBlockDragDrop } from '$lib/document/transcription/useBlockDragDrop.svelte';
+import { withCsrf } from '$lib/shared/cookies';
 
 type Props = {
 	documentId: string;
@@ -113,11 +114,14 @@ function handleDelete(blockId: string) {
 
 async function reorder(newOrder: string[]) {
 	try {
-		const res = await fetch(`/api/documents/${documentId}/transcription-blocks/reorder`, {
-			method: 'PUT',
-			headers: { 'Content-Type': 'application/json' },
-			body: JSON.stringify({ blockIds: newOrder })
-		});
+		const res = await fetch(
+			`/api/documents/${documentId}/transcription-blocks/reorder`,
+			withCsrf({
+				method: 'PUT',
+				headers: { 'Content-Type': 'application/json' },
+				body: JSON.stringify({ blockIds: newOrder })
+			})
+		);
 		if (!res.ok) return;
 		const updated = await res.json();
 		for (const b of updated) {

frontend/src/lib/document/transcription/useBlockAutoSave.svelte.ts

@@ -1,5 +1,6 @@
 import { SvelteMap } from 'svelte/reactivity';
 import type { PersonMention } from '$lib/shared/types';
+import { withCsrf } from '$lib/shared/cookies';
 
 export type SaveState = 'idle' | 'saving' | 'saved' | 'fading' | 'error';
 
@@ -116,12 +117,15 @@ export function createBlockAutoSave({ saveFn, documentId }: Options) {
 		for (const [blockId, text] of pendingTexts) {
 			const mentions = pendingMentions.get(blockId) ?? [];
 			clearDebounce(blockId);
-			void fetch(`/api/documents/${documentId}/transcription-blocks/${blockId}`, {
-				method: 'PUT',
-				headers: { 'Content-Type': 'application/json' },
-				body: JSON.stringify({ text, mentionedPersons: mentions }),
-				keepalive: true
-			});
+			void fetch(
+				`/api/documents/${documentId}/transcription-blocks/${blockId}`,
+				withCsrf({
+					method: 'PUT',
+					headers: { 'Content-Type': 'application/json' },
+					body: JSON.stringify({ text, mentionedPersons: mentions }),
+					keepalive: true
+				})
+			);
 			pendingTexts.delete(blockId);
 			pendingMentions.delete(blockId);
 		}

frontend/src/lib/document/transcription/useTranscriptionBlocks.svelte.ts

@@ -2,6 +2,7 @@
    lastEditedAt's $derived are scope-local to one computation; they're never
    stored on $state. */
 import type { TranscriptionBlockData, PersonMention } from '$lib/shared/types';
+import { makeCsrfFetch } from '$lib/shared/cookies';
 import { saveBlockWithConflictRetry } from './saveBlockWithConflictRetry';
 import { BlockConflictResolvedError } from './blockConflictMerge';
 
@@ -41,7 +42,7 @@ export function createTranscriptionBlocks(
 	options: TranscriptionBlocksOptions
 ): TranscriptionBlocksController {
 	const { documentId } = options;
-	const fetchImpl = options.fetchImpl ?? fetch;
+	const fetchImpl = makeCsrfFetch(options.fetchImpl ?? fetch);
 
 	let blocks = $state<TranscriptionBlockData[]>([]);
 	let annotationReloadKey = $state(0);

frontend/src/lib/ocr/OcrTrainingCard.svelte

@@ -2,6 +2,7 @@
 import TrainingHistory from './TrainingHistory.svelte';
 import { m } from '$lib/paraglide/messages.js';
 import type { TrainingRun } from '$lib/ocr/training.js';
+import { withCsrf } from '$lib/shared/cookies';
 
 interface TrainingInfo {
 	availableBlocks?: number;
@@ -33,7 +34,7 @@ async function startTraining() {
 	successMessage = null;
 	errorMessage = null;
 	try {
-		const res = await fetch('/api/ocr/train', { method: 'POST' });
+		const res = await fetch('/api/ocr/train', withCsrf({ method: 'POST' }));
 		if (res.ok) {
 			successMessage = m.training_success();
 			setTimeout(() => {

frontend/src/lib/ocr/SegmentationTrainingCard.svelte

@@ -2,6 +2,7 @@
 import TrainingHistory from './TrainingHistory.svelte';
 import { m } from '$lib/paraglide/messages.js';
 import type { TrainingRun } from '$lib/ocr/training.js';
+import { withCsrf } from '$lib/shared/cookies';
 
 interface TrainingInfo {
 	availableSegBlocks?: number;
@@ -27,7 +28,7 @@ async function startTraining() {
 	training = true;
 	successMessage = null;
 	try {
-		const res = await fetch('/api/ocr/segtrain', { method: 'POST' });
+		const res = await fetch('/api/ocr/segtrain', withCsrf({ method: 'POST' }));
 		if (res.ok) {
 			successMessage = m.training_success();
 			setTimeout(() => {

frontend/src/lib/shared/cookies.ts

@@ -1,3 +1,46 @@
+/**
+ * Reads the XSRF-TOKEN cookie set by Spring Security's CookieCsrfTokenRepository.
+ * Returns null outside the browser or when the cookie is absent.
+ */
+export function getCsrfToken(): string | null {
+	if (typeof document === 'undefined') return null;
+	const match = document.cookie.match(/(?:^|;\s*)XSRF-TOKEN=([^;]+)/);
+	return match ? decodeURIComponent(match[1]) : null;
+}
+
+/**
+ * Merges the X-XSRF-TOKEN header into a RequestInit so Spring Security's
+ * CSRF filter accepts the request. Safe to call server-side (no-op when the
+ * cookie is absent).
+ */
+export function withCsrf(init?: RequestInit): RequestInit {
+	const token = getCsrfToken();
+	if (!token) return init ?? {};
+	const headers = new Headers(init?.headers);
+	headers.set('X-XSRF-TOKEN', token);
+	return { ...init, headers };
+}
+
+/**
+ * Wraps a fetch implementation so that every state-mutating call (POST, PUT,
+ * PATCH, DELETE) automatically includes the X-XSRF-TOKEN header. GET/HEAD
+ * requests pass through unchanged.
+ *
+ * Used to CSRF-protect client-side hooks that accept an injectable fetchImpl.
+ * In unit tests the injected mock is wrapped but getCsrfToken() returns null
+ * (no browser cookie), so no header is added and existing test expectations
+ * are unaffected.
+ */
+export function makeCsrfFetch(inner: typeof fetch): typeof fetch {
+	return (input: RequestInfo | URL, init?: RequestInit): Promise<Response> => {
+		const method = (init?.method ?? 'GET').toUpperCase();
+		if (['POST', 'PUT', 'PATCH', 'DELETE'].includes(method)) {
+			return inner(input, withCsrf(init));
+		}
+		return inner(input, init);
+	};
+}
+
 /**
  * Extracts the fa_session cookie value from a list of Set-Cookie response headers.
  *