Handle errors and access gates

Every non-2xx SDK response throws BoardApiError. Branch on its typed guards and codes; messages remain presentation text rather than control flow.

Error contract

class BoardApiError extends Error {
  status: number;
  code: string;
  details?: unknown;
  requestId?: string;
  raw: unknown;
}

Use the narrowest matching guard, render its domain state, and rethrow unmatched failures: