[area523ufo]

Closes #974

Summary

Enhances StellarClient in src/lib/client.ts:

1. Exponential backoff retry for transient failures: Added withRetry() with configurable maxRetries and retryBaseDelayMs (baseDelay * 2^attempt) that automatically retries transient network / HTTP 429 / 5xx / tx_too_late errors while immediately surfacing non-retryable errors.
2. Horizon error code decoder: Added decodeHorizonError() and HORIZON_ERROR_MESSAGES mapping standard Stellar Horizon transaction & operation result codes (tx_failed, tx_bad_seq, tx_insufficient_balance, tx_insufficient_fee, op_underfunded, op_no_trust, op_line_full, etc.) to human-readable descriptions.
3. Request timeout handling: Added configurable requestTimeoutMs enforced on RPC calls.

Verification

• Expanded src/lib/__tests__/mock-client.test.ts with tests for retry backoff on transient failures, non-retryable error fast failure, timeout handling, and Horizon error code decoding.
• All unit tests pass via npx vitest run.