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, and the fee no longer
matched the approval screen's. The ETH balance is now stored exactly, and the
send screen's Current balance and the send-confirm screen's balance, fee,
reserve and insufficient-balance messages go through truncateAmountNeverZero(),
as the approval screen does. Token balances keep their six-decimal value: it is
also what 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