fix: the confirmation screen reads its balance on the network it is drawn for (closes #558)

ConfirmTx showed and checked the balance Send handed it, so a popup reopened onto a confirmation saved before a network switch stated the other network's balance and checked the amount against it.

Each time it is shown, ConfirmTx now reads the From address's ETH balance, and for a token send its token balance, from the node of the network it is drawn for, and gives that node to the fee estimate, which starts once the read is done. Until then the balance line says "Loading..." and Sign & Send stays disabled; a failed read is stated as unknown. Each message the read can bring has a box reserved from the first paint, so nothing moves when it returns.

Model: opus-5-5

src/shared/txValidation.js

@@ -111,8 +111,8 @@ function isValidAmount(amount, tokenDecimals = null) {
 
 // The most ETH a send can carry: the exact balance minus the fee reserve from
 // feeReserveWei(), as a decimal string, so validateTransfer() passes it with
-// exactly that reserve left behind. `ethBalance` is the exact decimal string
-// balances.js stores, never a rounded one. Null when the balance does not
+// exactly that reserve left behind. `ethBalance` is an exact decimal string as
+// formatEther() writes it, never a rounded one. Null when the balance does not
 // leave anything to send once the fee is paid, or when either input is
 // unusable.
 function maxEthAmount(ethBalance, feeWei) {