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/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.
  *