Recover channel moves from interruption and retries #429

Moves between this host and a hosted cell (#194, #181) work for idle channels with an exclusive cutover. What's missing is recovery when a move is interrupted: a crash, a lost reply, network loss, or the same command sent again during export, import or cutover.

• A failed or interrupted move leaves the source usable, or offers a clear way to resume or roll back.
• Retrying a move can never leave two active owners of the channel.
• The UI says what state the move was left in (it builds on fix(app): show a move's progress instead of Try again while it runs #375's progress).

Not in scope unless needed: moving busy channels, channels over 32 MiB, and moves across pi versions. Backup and restore is #178.