bug: spending confirm swipe can pay a different amount than the screen

[ovitrif]

Twin: synonymdev/bitkit-ios#783
Refs: #1323 (comment)

What happened?

On transfer to spending, confirm shows a total from a funding plan built before the Blocktank order exists. Since #1247 the order is created on swipe. The swipe then rebuilds the plan against the order: its address and fee, fresh balances and a fresh fee rate. It pays that rebuilt plan without showing it, so the network fee, the send-all choice and the total that leaves can differ from the screen.

ensureSpendingOrder stops the payment only when the order fee is higher than the fee on screen. Any other difference pays on the same swipe.

Raised by @jvsena42 in the review of #1323. #1247 is not in 2.5.0, so this reaches users with 2.6.0 unless fixed first.

Expected behavior

When the rebuilt amount differs from the amount on screen, the screen shows the new amount and the payment waits for another swipe.

Steps to Reproduce

1. Open transfer to spending and reach confirm.
2. Leave it until the fast fee rate changes.
3. Swipe to pay.
4. Compare the amount that leaves with the total that was on screen.

Logs / Screenshots / Recordings

N/A

Bitkit Version

master since #1247 (after 2.5.0)

Device / OS

Android. iOS has the same rebuild: synonymdev/bitkit-ios#783.

Reproducibility

When the plan rebuilt at swipe differs from the plan on screen.

[ovitrif]

Refined behavior, decided on 28 Sep: a swipe pays at once when the plan rebuilt for the order costs the same as or less than the total on screen, so the user never pays more than shown. When it costs more, nothing is paid, Spending Confirm shows the new amounts, a "Fees changed" toast says how much higher the fees are (in the user's primary display unit) and why, and the next swipe pays. Implemented in #1367; iOS follows in synonymdev/bitkit-ios#825.