Fix submit transport retry idempotency

[telemarkdigital-publisher]

Summary

• prepare one signed submit payload and reuse it for transport retries so timeout/all-endpoint retry paths do not reserve a fresh sequence
• send a stable per-submit idempotency key in the request body and Idempotency-Key header
• poll the submission status endpoint after indeterminate transport failures before retrying, while preserving BadSequence refresh/re-sign behavior

Closes #86

Verification

• npm test -- --runInBand src/tests/contract.test.ts
• npm test -- --runInBand
• npm run typecheck
• npm run lint (passes with existing warnings in src/account/tests/balances.test.ts)
• npm run build