refactor(frontend): replace all as-unknown-as error casts with extractErrorCode

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

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

@@ -1,5 +1,5 @@
 import { redirect } from '@sveltejs/kit';
-import { createApiClient } from '$lib/shared/api.server';
+import { createApiClient, extractErrorCode } from '$lib/shared/api.server';
 import { getErrorMessage } from '$lib/shared/errors';
 import type { components } from '$lib/generated/api';
 
@@ -103,8 +103,7 @@ export async function load({ url, fetch }) {
 	}
 
 	const errorMessage: string | null = !result.response.ok
-		? (getErrorMessage((result.error as unknown as { code?: string })?.code) ??
-			'Daten konnten nicht geladen werden.')
+		? (getErrorMessage(extractErrorCode(result.error)) ?? 'Daten konnten nicht geladen werden.')
 		: null;
 
 	return {