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/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);