Add offboard/send_onchain wallet action

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Summary by CodeRabbit

• New Features
  • Offboarding is now crash-safe and resumable: progress is checkpointed and automatically continued on the next sync.
  • Added the ability to inspect and drive pending offboards via pending_offboards() / sync_pending_offboards().
  • Introduced configurable timing for periodic offboard checks and how long missing offboard txs are treated as “not lost”.
• Bug Fixes
  • If offboard finish/replies are lost, retries deterministically replay the same signed transaction.
  • If an offboard tx disappears, it’s rebroadcast during the grace period and then reported lost; failed offboards release funds and mark movements as failed.
• Breaking Changes
  • Persistence integrations must implement get_wallet_vtxos; legacy pending-offboard persistence is removed.