fix: handle dust change in transfers and sends

[ben-kaufman]

Description

This PR fixes accurate full-balance sending for Blocktank channel purchases:

1. Two-pass fee estimation in SpendingAmount ensures LSP fee calculation matches actual order creation (fixes off-by-few-sats errors where max showed higher than actually sendable)
2. Dust detection in SpendingConfirm uses sendAll when change would be dust, avoiding unspendable outputs
3. sendAll support for normal on-chain sends when change would be below dust limit
4. Safety validation ensures max sendable amount hasn't changed due to fee rate changes before executing transfer

Technical Details

The core issue was that lspBalance values depend on clientBalanceSat, so fee estimation with maxSendable produced different fees than actual order creation with clientBalance. The two-pass estimation now:

1. First pass: estimates with maxSendable to get approximate clientBalance
2. Second pass: recalculates lspBalance using approxClientBalance to match what order creation will use

Linked Issues/Tasks

Fixes MAX transfer to spending failing with insufficient funds error when calculated change would be dust.

How to Test

MAX Transfer to Spending

1. Have on-chain balance (e.g., 5000 sats)
2. Go to Transfer > To Spending
3. Press MAX button
4. Tap Continue - amounts should add up correctly on confirm screen
5. Swipe to transfer - should succeed without "Insufficient funds" error

Near-MAX Normal Send

1. Have on-chain balance
2. Scan/enter an on-chain address
3. Enter an amount that leaves change below dust limit (~547 sats)
4. Confirm screen should show and swipe should work
5. Transaction should succeed

[piotr-iohk]

@ben-kaufman should this fix #200?
On my side I still see it

[ben-kaufman]

I actually couldn't reproduce #200
The issue I got was insufficient funds, not coin selection, and that's what I fixed for. Can you please share logs for the coin selection error?

[piotr-iohk]

I actually couldn't reproduce #200 The issue I got was insufficient funds, not coin selection, and that's what I fixed for. Can you please share logs for the coin selection error?

bitkit_logs_2026-01-28_17-08-13.zip

[claude]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.

Note: The PR introduces Bitcoin/Lightning operations in view files, which technically violates CLAUDE.md's guideline ("All Bitcoin/Lightning operations belong in the service layer, never in views"). However, this pattern is consistent with 35+ existing files in the codebase, and addressing it would require an architectural refactoring beyond the scope of this PR.

[piotr-iohk]

The exact case from #200 is still failing for me here giving "Coin selection error". Interestingly it passes on Android.
The cases below work fine here. Near-MAX Normal Send - works fine for 100k sats, the #200 case uses 100 000 000 sats and then it fails with "Coin selection error":

MAX Transfer to Spending

1. Have on-chain balance (e.g., 5000 sats)
2. Go to Transfer > To Spending
3. Press MAX button
4. Tap Continue - amounts should add up correctly on confirm screen
5. Swipe to transfer - should succeed without "Insufficient funds" error

Near-MAX Normal Send

1. Have on-chain balance
2. Scan/enter an on-chain address
3. Enter an amount that leaves change below dust limit (~547 sats)
4. Confirm screen should show and swipe should work
5. Transaction should succeed

Note the MAX Transfer to Spending case fails on Android, created issue for it: synonymdev/bitkit-android#745

[piotr-iohk]

Approving since #200 issue was not originally intended to be fixed here, can be addressed separately.