bark: Add board wallet action

Board registration is attempted when boards are being driven through the Confirming phase. I also preserved the TODO around cancelling pending exits for a board that eventually succeeds.

Summary by CodeRabbit

• New Features

  • Boarding now runs as a crash-safe, durable wallet checkpoint, allowing in-progress boards to resume automatically after interruptions.
  • In-flight boards are migrated automatically from the legacy storage into the new checkpoint system.
  • Board lifecycle progress is tracked and advanced by the wallet until completion or parking.

• Bug Fixes

  • More reliable recovery: funding is rebroadcast if it drops from the mempool, and registration retries no longer lose the locked VTXO state.
  • Near-expiry handling can initiate an idempotent exit while continuing retries for registration.

• Tests

  • Updated and expanded board flow and migration coverage for the new checkpoint model.