fix: show balances and fees below 0.000001 ETH as nonzero on the send screens (closes #343)

The stored ETH balance and the send-confirm screen's fee were each cut to six
decimal places, so a value below 0.000001 read as 0.0. The ETH balance is now
stored exactly, and the send and send-confirm screens' balances, reserve and
insufficient-balance messages go through truncateAmountNeverZero(). Both the
send-confirm and approval screens render the fee through formatFee(), which
prices the exact fee in USD, so the same fee reads the same on both. Token
balances keep their six-decimal value: it also leaves dust off the balance
list, and keeps the string within the 18 decimals the balance check reads.

Model: opus-5-5

tests/e2e/run.js

@@ -1414,9 +1414,10 @@ test("a rejected dust threshold shifts no layout (#233)", async (env) => {
 // on opposite sides of the reserve while sitting on the same side of the
 // estimate.
 
-// The balance the funded fixture serves, and the amounts sent against it.
+// The balance the funded fixture serves, as the Send and confirmation screens
+// show it, and the amounts sent against it.
 const FUNDED_ETH_WEI = 10n ** 18n;
-const FUNDED_ETH_TEXT = "1.0";
+const FUNDED_ETH_TEXT = "1.0000";
 const COMFORTABLE_AMOUNT = "0.1";
 const OVER_BALANCE_AMOUNT = "2.0";
 
@@ -1430,7 +1431,7 @@ const GAP_AMOUNT = formatEther(FUNDED_ETH_WEI - FEE_ESTIMATE_WEI);
 // fee test: it covers the expected cost to the wei and falls short of the
 // reserve, so the same swap flips this assertion too — through a different
 // balance and a different message than the ETH path uses.
-const TOKEN_BALANCE_TEXT = "1.5";
+const TOKEN_BALANCE_TEXT = "1.5000";
 const TOKEN_AMOUNT = "0.25";
 const OVER_TOKEN_AMOUNT = "9.0";
 const FEE_ONLY_ETH_WEI = FEE_ESTIMATE_WEI;
@@ -1439,16 +1440,15 @@ function toHexWei(wei) {
     return "0x" + wei.toString(16);
 }
 
-// A fee in wei as the confirmation screen writes it. Deliberately a second
-// implementation of formatFeeEth() from src/popup/views/confirmTx.js rather
-// than an import of it: that module pulls in the whole popup and cannot be
-// required outside a browser, and asserting against an independent rendering
-// is stronger than asserting a function equals itself.
+// A fee in wei as the confirmation screen writes it: truncated to four decimal
+// places (README.md, Display Consistency). Deliberately a second
+// implementation rather than an import of src/shared/amountDisplay.js:
+// asserting against an independent rendering is stronger than asserting a
+// function equals itself. The fixture's fees are above 0.0001 ETH, so the
+// nonzero floor never applies here.
 function feeEth(wei) {
-    const parts = formatEther(wei).split(".");
-    const dec =
-        parts.length > 1 ? parts[1].slice(0, 6).replace(/0+$/, "") || "0" : "0";
-    return parts[0] + "." + dec + " ETH";
+    const [whole, frac = ""] = formatEther(wei).split(".");
+    return whole + "." + (frac + "0000").slice(0, 4) + " ETH";
 }
 
 // What the confirmation screen is showing right now, read out of the DOM in
@@ -1505,11 +1505,10 @@ async function backToAddress(page) {
 
 // Drive the popup to the confirmation screen for one send.
 //
-// It waits for the send screen to be showing `balance` before filling
-// anything in. That figure is the exact number the spend gate compares
-// against, so waiting for it — rather than for a refresh to have probably
-// landed — is what keeps every assertion below deterministic after a
-// fixture change.
+// It waits for the send screen to be showing `balance`, the fixture's balance
+// as that screen displays it, before filling anything in. Waiting for it —
+// rather than for a refresh to have probably landed — is what keeps every
+// assertion below deterministic after a fixture change.
 async function goToConfirm(page, { token, balance, amount }) {
     await backToAddress(page);
     await page.click("#btn-send");