Collaborative offboard: move the ark balance back on-chain

PrepareOffboard/FinishOffboard against captaind — per-input ownership
attestations, connector-based musig forfeits (with the deterministic
fanout tx when there is more than one input), and fee math that mirrors
the server bit-for-bit (weight fee rounds up, ppm-by-expiry rounds
down) since it rejects any other net amount. The receive pane gains a
'Move back on-chain' button that withdraws everything to the wallet's
own fresh address; the incoming tx is labeled 'Ark withdrawal' in
History and the ark side records the movement. Crash-safe: the txid
persists before FinishOffboard and resume asks GetVtxoStatus whether
the forfeits landed (the server broadcasts the tx itself).

Also fixes boarding to applySentTx after broadcasting the funding tx —
without it the spent coin lingered in the utxo set (a second board
double-spent it), and the post-board scan() could resurrect it by
racing the explorer's indexing.

Co-Authored-By: Claude Fable 5 <[email protected]>

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