refactor(frontend): add ApiError interface and extractErrorCode helper

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

frontend/src/lib/shared/api.server.ts

@@ -23,3 +23,12 @@ export function createApiClient(fetch: typeof globalThis.fetch) {
 		fetch
 	});
 }
+
+export interface ApiError {
+	code?: string;
+	message?: string;
+}
+
+export function extractErrorCode(error: unknown): string | undefined {
+	return (error as ApiError | undefined)?.code;
+}