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 screen the popup reopened onto stated the balance as Send knew it, not as it is on the network the screen is drawn for.

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 reviewedNetworkProvider() picks for the network the transaction was reviewed for, the node the fee estimate and recipient checks ask. The fee estimate starts once the read returns. 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) {