feat: handle Horizon 429 rate limiting with backoff

[joshuaodoh122-hub]

• Add retryWithBackoff utility that reads Retry-After header on 429 responses and waits the exact number of seconds Horizon asks before retrying. Other retriable errors (5xx, network) use full-jitter exponential back-off (base 500ms, cap 30s, up to maxRetries times).
• Integrate retryWithBackoff into useQuery so all read hooks benefit automatically. Default: up to 3 retries on 429 only (retryOn5xx is opt-in to avoid breaking existing error-handling behaviour).
• Expose rateLimitedUntilRef from useQuery so useBalance can inspect it.
• Update useBalance polling to skip interval ticks while a 429 rate-limit window is still active, preventing the fixed-interval hammer that worsens the rate-limit situation.
• Add maxRetries option to useAccount, useAsset, useBalance, useClaimableBalance, usePayments, and useTransaction so consumers can tune retry behaviour per hook.
• Add 32 unit tests covering isRetriable, getRetryAfterMs, computeBackoffDelay, and retryWithBackoff (including abort-signal cancellation and the retryOn5xx flag).

Closes #193

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