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, so a value below 0.000001 read as zero. Balances are
now stored exactly, whatever decimals a token declares; the balance check reads
a token balance to its first 18 places, the most an amount can have. A token
holding below 0.000001 is still left off the lists as dust, except for a token
the user tracks. 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. The balance lists still round with
toFixed(4).

Model: opus-5-5

src/shared/balances.js

@@ -52,19 +52,13 @@ function requireNetworkId(networkId) {
     return net;
 }
 
-function formatBalance(wei) {
-    const eth = formatEther(wei);
-    const parts = eth.split(".");
-    if (parts.length === 1) return eth + ".0";
-    const dec = parts[1].slice(0, 6).replace(/0+$/, "") || "0";
-    return parts[0] + "." + dec;
-}
-
+// A token balance as an exact decimal string, never cut: a cut stores a small
+// nonzero holding as zero.
 function formatTokenBalance(raw, decimals) {
     const val = formatUnits(raw, decimals);
     const parts = val.split(".");
     if (parts.length === 1) return val + ".0";
-    const dec = parts[1].slice(0, 6).replace(/0+$/, "") || "0";
+    const dec = parts[1].replace(/0+$/, "") || "0";
     return parts[0] + "." + dec;
 }
 
@@ -149,11 +143,7 @@ async function fetchTokenBalances(address, blockscoutUrl, trackedTokens) {
             const scale = known !== null ? known : decimals;
             // null is a holding of an amount that cannot be stated, which is
             // not the same as a holding of zero, and must never render as one.
-            // With a scale, the display filter proper applies: a balance that
-            // rounds to zero at six places is dust and is not listed. Without
-            // one there is no such judgement to make, and the row is kept.
             const bal = scale === null ? null : formatTokenBalance(raw, scale);
-            if (bal === "0.0") continue;
             // null means the explorer reported no count, which is not the
             // same as a count of zero.  This gate is not the low-holder
             // display filter: it has no user-facing off switch and governs
@@ -172,6 +162,19 @@ async function fetchTokenBalances(address, blockscoutUrl, trackedTokens) {
             // Skip spam tokens the user never asked to see
             if (!isKnown && !isTracked && !hasEnoughHolders) continue;
 
+            // Skip dust: a holding below 0.000001 of a token the user does not
+            // track. A tracked token keeps it, so the Send and confirmation
+            // screens show that holding and check the send against it rather
+            // than against zero. Without a scale there is no such judgement to
+            // make, and the row is kept.
+            if (
+                !isTracked &&
+                scale !== null &&
+                raw * 1000000n < 10n ** BigInt(scale)
+            ) {
+                continue;
+            }
+
             // Skip tokens spoofing a known symbol from a different address.
             // Every row here is an ERC-20 the explorer reported, so it has a
             // contract address; the native ETH balance is fetched over RPC in
@@ -221,7 +224,9 @@ async function refreshBalances(
                 provider
                     .getBalance(addr.address)
                     .then((bal) => {
-                        addr.balance = formatBalance(bal);
+                        // Exact, never cut: a cut here stores a small nonzero
+                        // balance as zero.
+                        addr.balance = formatEther(bal);
                         log.debugf("ETH balance", addr.address, addr.balance);
                     })
                     .catch((e) => {

src/shared/txValidation.js

@@ -139,7 +139,15 @@ function validateTransfer({
     const feeFp = known ? feeWei : null;
 
     if (isErc20) {
-        const tokenFp = toFixedPoint(tokenBalance) ?? 0n;
+        // A token can declare more than 18 decimals, and its balance is
+        // stored with all of them. Only the first 18 places (SCALE_DECIMALS)
+        // are read: an amount with more was refused above, so the places
+        // after them cannot decide whether the amount fits.
+        const tokenText =
+            typeof tokenBalance === "string"
+                ? tokenBalance.replace(/(\.\d{18})\d+$/, "$1")
+                : tokenBalance;
+        const tokenFp = toFixedPoint(tokenText) ?? 0n;
         if (amountFp > tokenFp) codes.push(CODES.INSUFFICIENT_TOKEN);
         if (feeFp !== null && feeFp > ethFp) {
             codes.push(CODES.INSUFFICIENT_ETH_FOR_FEE);