On-chain send can pay a different fee rate than Confirm showed #1722

What happens

The on-chain send wrapper in AppViewModel doesn't pass the fee rates that Confirm used:

// AppViewModel.sendOnchain (master)
): Result<OnchainSendOutcome> = lightningRepo.sendOnChain(
    address = address,
    sats = amount,
    speed = _sendUiState.value.speed,
    utxosToSpend = _sendUiState.value.selectedUtxos,
    ...
)

LightningRepo.sendOnChain takes a feeRates: FeeRates? = null parameter. With it left null, it resolves getFees() again when the user swipes. The Confirm screen, the fee estimate and UTXO selection all used SendUiState.feeRates, captured when the send flow started. If Blocktank's rates move between review and swipe, the transaction pays a different fee rate from the one the user approved, higher or lower.

Expected

The broadcast uses the fee rates the user saw on Confirm.

Suggested fix

Pass feeRates = _sendUiState.value.feeRates in the wrapper. A unit test in AppViewModelSendFlowTest can check that sendOnChain receives the SendUiState rates and not a fresh getFees() result.

Found while reviewing #1701. That PR's ordinary send uses the selected cached fee test passes through this path but doesn't cover rates changing between review and swipe.