fix: every balance is cut to four decimals (closes #757)

The balance lists and Home's ETH line rounded with toFixed(4), so 0.99996
ETH read 1.0000 there and 0.9999 on Send. Send's current balance and
ConfirmTx's balance line kept the nonzero floor, so 0.00004 USDC read
0.0000 in the lists and 0.00004 on Send. All four now cut the stored
decimal string with truncateAmount(), as the history rows do.
balanceLine() takes the stored balance string instead of a parsed number,
and escapes the quantity cell. The amounts, fees and allowances a screen
asks the user to approve keep the floor of truncateAmountNeverZero().

Disclosure: for a holding below 0.0001, ConfirmTx shows the amount sent
beside a balance that reads 0.0000.

Model: opus-5-5

src/popup/views/confirmTx.js

@@ -43,9 +43,12 @@ const {
     transferAmountUnits,
 } = require("../../shared/transferAmount");
 const { assertWithinCeilings } = require("../../shared/approvalVerify");
-// The amount, the balance line and the fee reserve go through it, as the
-// approval screen's amounts do.
-const { truncateAmountNeverZero } = require("../../shared/amountDisplay");
+// The amount and the fee reserve keep the nonzero floor, as the approval
+// screen's amounts do. The balance line is cut as the balance lists cut it.
+const {
+    truncateAmount,
+    truncateAmountNeverZero,
+} = require("../../shared/amountDisplay");
 const {
     CODES,
     FEE_PENDING,
@@ -160,7 +163,7 @@ function renderBalance(txInfo) {
     const price = getPrice(txInfo.token, txInfo.chainId);
     const balUsd = price ? parseFloat(bal) * price : null;
     $("confirm-balance").textContent = valueWithUsd(
-        truncateAmountNeverZero(bal) + " " + symbol,
+        truncateAmount(bal) + " " + symbol,
         balUsd,
     );
 }