fix(frontend): add extractErrorCode to all api.server vi.mock factories

All route spec files that mock $lib/shared/api.server were missing
extractErrorCode from the mock factory, causing a vitest "No export defined"
error after the refactor introduced the new export.

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

frontend/src/routes/briefwechsel/page.server.spec.ts

@@ -1,7 +1,10 @@
 import { describe, expect, it, vi, beforeEach } from 'vitest';
 import { load } from './+page.server';
 
-vi.mock('$lib/shared/api.server', () => ({ createApiClient: vi.fn() }));
+vi.mock('$lib/shared/api.server', () => ({
+	createApiClient: vi.fn(),
+	extractErrorCode: (e: unknown) => (e as { code?: string } | undefined)?.code
+}));
 vi.mock('$lib/shared/errors', () => ({
 	getErrorMessage: (code: string) => code ?? 'Unknown error'
 }));