feat(frontend): add CSRF injection, rate-limit i18n, and 429 login handling

- handleFetch injects X-XSRF-TOKEN + XSRF-TOKEN cookie on all mutating
  backend API requests (double-submit cookie pattern); generates a fresh
  UUID when no XSRF-TOKEN cookie exists yet
- ErrorCode union gains CSRF_TOKEN_MISSING and TOO_MANY_LOGIN_ATTEMPTS;
  getErrorMessage maps both to i18n keys
- de/en/es messages add error_csrf_token_missing and
  error_too_many_login_attempts translations
- Login action maps HTTP 429 to fail(429, { ..., rateLimited: true });
  page shows a muted clock icon with aria-invalid on rate-limit errors

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

frontend/messages/de.json

@@ -19,6 +19,8 @@
 	"error_session_expired_explainer": "Aus Sicherheitsgründen werden Sitzungen nach 8 Stunden Inaktivität automatisch beendet.",
 	"error_unauthorized": "Sie sind nicht angemeldet.",
 	"error_forbidden": "Sie haben keine Berechtigung für diese Aktion.",
+	"error_csrf_token_missing": "Sitzungsfehler. Bitte laden Sie die Seite neu.",
+	"error_too_many_login_attempts": "Zu viele Anmeldeversuche. Bitte versuchen Sie es später erneut.",
 	"error_validation_error": "Die Eingabe ist ungültig.",
 	"error_internal_error": "Ein unerwarteter Fehler ist aufgetreten.",
 	"nav_documents": "Dokumente",

frontend/messages/en.json

@@ -19,6 +19,8 @@
 	"error_session_expired_explainer": "For security reasons, sessions are automatically ended after 8 hours of inactivity.",
 	"error_unauthorized": "You are not logged in.",
 	"error_forbidden": "You do not have permission for this action.",
+	"error_csrf_token_missing": "Session error. Please reload the page.",
+	"error_too_many_login_attempts": "Too many login attempts. Please try again later.",
 	"error_validation_error": "The input is invalid.",
 	"error_internal_error": "An unexpected error occurred.",
 	"nav_documents": "Documents",

frontend/messages/es.json

@@ -19,6 +19,8 @@
 	"error_session_expired_explainer": "Por razones de seguridad, las sesiones se terminan automáticamente tras 8 horas de inactividad.",
 	"error_unauthorized": "No ha iniciado sesión.",
 	"error_forbidden": "No tiene permiso para realizar esta acción.",
+	"error_csrf_token_missing": "Error de sesión. Recargue la página.",
+	"error_too_many_login_attempts": "Demasiados intentos. Por favor, inténtelo más tarde.",
 	"error_validation_error": "La entrada no es válida.",
 	"error_internal_error": "Se ha producido un error inesperado.",
 	"nav_documents": "Documentos",

frontend/src/hooks.server.ts

@@ -96,42 +96,58 @@ const userGroup: Handle = async ({ event, resolve }) => {
 	return resolve(event);
 };
 
+const MUTATING_METHODS = new Set(['POST', 'PUT', 'PATCH', 'DELETE']);
+
+// Auth endpoints that establish/check their own credentials — skip fa_session injection
+// but still need CSRF tokens on mutating requests.
+const PUBLIC_API_PATHS = [
+	'/api/auth/login',
+	'/api/auth/logout',
+	'/api/auth/forgot-password',
+	'/api/auth/reset-password',
+	'/api/auth/invite/',
+	'/api/auth/register'
+];
+
 export const handleFetch: HandleFetch = async ({ event, request, fetch }) => {
 	const apiUrl = env.API_INTERNAL_URL || 'http://localhost:8080';
 	const isApi = request.url.startsWith(apiUrl) || request.url.includes('/api/');
 
-	if (isApi) {
-		// Auth endpoints that establish/check their own credentials manage cookies themselves;
-		// don't double-inject a stale fa_session.
-		const PUBLIC_API_PATHS = [
-			'/api/auth/login',
-			'/api/auth/logout',
-			'/api/auth/forgot-password',
-			'/api/auth/reset-password',
-			'/api/auth/invite/',
-			'/api/auth/register'
-		];
-		if (PUBLIC_API_PATHS.some((p) => request.url.includes(p))) {
-			return fetch(request);
-		}
+	if (!isApi) return fetch(request);
 
-		const sessionId = event.cookies.get('fa_session');
-		if (!sessionId) {
-			return new Response('Unauthorized', { status: 401 });
-		}
+	const isMutating = MUTATING_METHODS.has(request.method);
+	const isPublicAuthApi = PUBLIC_API_PATHS.some((p) => request.url.includes(p));
 
-		// Clone first so the body stream is preserved on the new Request.
-		const cloned = request.clone();
-		const modified = new Request(cloned, {
-			headers: {
-				...Object.fromEntries(cloned.headers),
-				Cookie: `fa_session=${sessionId}`
-			}
-		});
-		return fetch(modified);
+	const sessionId = !isPublicAuthApi ? event.cookies.get('fa_session') : null;
+	if (!isPublicAuthApi && !sessionId) {
+		return new Response('Unauthorized', { status: 401 });
 	}
 
-	return fetch(request);
+	// Read the browser's XSRF-TOKEN cookie; fall back to a fresh UUID for the
+	// double-submit cookie pattern (both cookie and header must match — no server secret).
+	const xsrfToken = isMutating ? (event.cookies.get('XSRF-TOKEN') ?? crypto.randomUUID()) : null;
+
+	const cookieParts: string[] = [];
+	if (sessionId) cookieParts.push(`fa_session=${sessionId}`);
+	if (xsrfToken) cookieParts.push(`XSRF-TOKEN=${xsrfToken}`);
+
+	if (cookieParts.length === 0 && !xsrfToken) {
+		return fetch(request);
+	}
+
+	// Clone first so the body stream is preserved on the new Request.
+	const cloned = request.clone();
+	const extraHeaders: Record<string, string> = {};
+	if (cookieParts.length > 0) extraHeaders['Cookie'] = cookieParts.join('; ');
+	if (xsrfToken) extraHeaders['X-XSRF-TOKEN'] = xsrfToken;
+
+	const modified = new Request(cloned, {
+		headers: {
+			...Object.fromEntries(cloned.headers),
+			...extraHeaders
+		}
+	});
+	return fetch(modified);
 };
 
 export const handle = sequence(userGroup, handleAuth, handleLocaleDetection, handleParaglide);

frontend/src/lib/shared/errors.ts

@@ -49,6 +49,8 @@ export type ErrorCode =
 	| 'MISSING_CREDENTIALS'
 	| 'UNAUTHORIZED'
 	| 'FORBIDDEN'
+	| 'CSRF_TOKEN_MISSING'
+	| 'TOO_MANY_LOGIN_ATTEMPTS'
 	| 'VALIDATION_ERROR'
 	| 'BATCH_TOO_LARGE'
 	| 'BULK_EDIT_TOO_MANY_IDS'
@@ -166,6 +168,10 @@ export function getErrorMessage(code: ErrorCode | string | undefined): string {
 			return m.error_unauthorized();
 		case 'FORBIDDEN':
 			return m.error_forbidden();
+		case 'CSRF_TOKEN_MISSING':
+			return m.error_csrf_token_missing();
+		case 'TOO_MANY_LOGIN_ATTEMPTS':
+			return m.error_too_many_login_attempts();
 		case 'VALIDATION_ERROR':
 			return m.error_validation_error();
 		case 'BATCH_TOO_LARGE':

frontend/src/routes/login/+page.server.ts

@@ -45,6 +45,10 @@ export const actions = {
 			return fail(401, { error: getErrorMessage(code) });
 		}
 
+		if (response.status === 429) {
+			return fail(429, { error: getErrorMessage('TOO_MANY_LOGIN_ATTEMPTS'), rateLimited: true });
+		}
+
 		if (!response.ok) {
 			return fail(response.status, { error: getErrorMessage('INTERNAL_ERROR') });
 		}

frontend/src/routes/login/+page.svelte

@@ -7,7 +7,7 @@ let {
 	form
 }: {
 	data: { registered: boolean; reason?: string | null };
-	form?: { error?: string; success?: boolean };
+	form?: { error?: string; rateLimited?: boolean; success?: boolean };
 } = $props();
 </script>
 
@@ -106,7 +106,31 @@ let {
 					</div>
 
 					{#if form?.error}
-						<div class="text-center font-sans text-xs font-medium text-red-600">{form.error}</div>
+						{#if form?.rateLimited}
+							<div
+								aria-invalid="true"
+								role="alert"
+								class="flex items-center gap-2 font-sans text-xs font-medium text-red-600"
+							>
+								<svg
+									aria-hidden="true"
+									viewBox="0 0 24 24"
+									fill="none"
+									stroke="currentColor"
+									stroke-width="1.5"
+									class="h-4 w-4 shrink-0 text-ink-3"
+								>
+									<path
+										stroke-linecap="round"
+										stroke-linejoin="round"
+										d="M12 6v6h4.5m4.5 0a9 9 0 1 1-18 0 9 9 0 0 1 18 0Z"
+									/>
+								</svg>
+								<span>{form.error}</span>
+							</div>
+						{:else}
+							<div class="text-center font-sans text-xs font-medium text-red-600">{form.error}</div>
+						{/if}
 					{/if}
 
 					<button

frontend/src/routes/login/page.server.test.ts

@@ -100,6 +100,25 @@ describe('login action', () => {
 		expect(cookies.delete).toHaveBeenCalledWith('auth_token', { path: '/' });
 	});
 
+	it('returns 429 with rateLimited=true when the backend rate-limits the request', async () => {
+		const mockFetch = vi.fn().mockResolvedValue(
+			new Response(JSON.stringify({ code: 'TOO_MANY_LOGIN_ATTEMPTS' }), {
+				status: 429,
+				headers: { 'Content-Type': 'application/json' }
+			})
+		);
+
+		const result = await (actions as ActionsRecord).login({
+			request: makeRequest({ email: 'a@b.de', password: 'pw' }),
+			cookies: makeCookies(),
+			fetch: mockFetch,
+			url: new URL('http://localhost/login')
+		} as never);
+
+		expect((result as { status: number }).status).toBe(429);
+		expect((result as { data: { rateLimited: boolean } }).data.rateLimited).toBe(true);
+	});
+
 	it('returns 500 when backend response omits fa_session cookie', async () => {
 		const mockFetch = vi.fn().mockResolvedValue(new Response('{}', { status: 200 }));
 		const cookies = makeCookies();