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

The stored ETH and token balances and the send-confirm screen's fee were each
cut to six decimal places, and a token holding cut to zero was dropped, so a
value below 0.000001 read as zero. Balances are now stored exactly, whatever
decimals a token declares, and every nonzero token holding is kept; the balance
check reads a token balance to its first 18 places. The balance lists, the
send-screen token selector, the address total and the remove-address warning
leave out a holding below 0.000001 themselves, through isBelowOneMillionth().
The send and send-confirm screens' balances, reserve and insufficient-balance
messages go through truncateAmountNeverZero(). The send-confirm and approval
screens both render the fee through formatFee(), which prices the exact fee in
USD.

Model: opus-5-5

src/popup/views/approval.js

@@ -8,6 +8,7 @@ const {
     renderAddressHtml,
     attachCopyHandlers,
     onViewLeave,
+    formatFee,
 } = require("./helpers");
 const { state, saveState } = require("../../shared/state");
 const { networkByChainId } = require("../../shared/networks");
@@ -207,7 +208,7 @@ function showPhishingWarning(elementId, isPhishing) {
 // and the nonce. The background compares every one of them against the signed
 // artifact, so every one of them has to be on the screen — a number that is
 // verified but never displayed is verified against nothing the user agreed to.
-function showTxFee(approvedTx, ethPrice) {
+function showTxFee(approvedTx) {
     const network = networkByChainId(approvedTx.chainId);
     $("approve-tx-network").textContent = network
         ? network.name
@@ -215,12 +216,9 @@ function showTxFee(approvedTx, ethPrice) {
 
     const gasLimit = BigInt(approvedTx.gasLimit);
     const feePerGas = BigInt(approvedTx.maxFeePerGas || approvedTx.gasPrice);
-    const maxFeeEth = formatTxValue(formatEther(gasLimit * feePerGas));
-    const usdStr = formatUsd(
-        ethPrice ? parseFloat(maxFeeEth) * ethPrice : null,
-    );
-    $("approve-tx-fee").textContent =
-        maxFeeEth + " ETH" + (usdStr ? " (" + usdStr + ")" : "");
+    // Through formatFee(), as the confirmation screen's fee is, so the same
+    // fee reads the same on both.
+    $("approve-tx-fee").textContent = formatFee(gasLimit * feePerGas);
 
     let detail =
         gasLimit.toString() +
@@ -332,7 +330,7 @@ function showTxApproval(details) {
     $("approve-tx-value").textContent =
         ethValueFormatted + " ETH" + (usdStr ? " (" + usdStr + ")" : "");
 
-    showTxFee(approvedTx, ethPrice);
+    showTxFee(approvedTx);
 
     // Decode calldata (reuse decoded from above)
     const decodedEl = $("approve-tx-decoded");