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/amountDisplay.js

@@ -6,10 +6,10 @@
 // (`src/shared/uniswap.js`) — and a fix applied to one of them left the other
 // two showing a different number for the same value.
 //
-// The two functions below are the two policies, not two implementations of
-// one: summary lists truncate, and the screens that state what is being
-// authorized truncate with a floor. Keeping them adjacent is the point, so a
-// change to the rule cannot reach one screen and miss another.
+// The two truncation functions below are the two policies, not two
+// implementations of one: summary lists truncate, and the screens that state
+// what is being authorized truncate with a floor. Keeping them adjacent is the
+// point, so a change to the rule cannot reach one screen and miss another.
 
 // Truncate to exactly four decimal places. Truncation, never rounding: an
 // amount must never be displayed as larger than it is, so 0.99999 stays
@@ -43,4 +43,18 @@ function truncateAmountNeverZero(val) {
     return parts[0] + "." + parts[1].slice(0, sig + 1);
 }
 
-module.exports = { truncateAmount, truncateAmountNeverZero };
+// Whether a stored token balance is a holding below 0.000001. The balance
+// lists, the send-screen token selector, the address total and the
+// remove-address warning leave such a holding out; the Send and confirmation
+// screens show it when its token is the one being sent. Exact, because
+// src/shared/balances.js stores plain decimal digits: below 0.000001 the
+// balance reads "0.000000" and then more digits.
+function isBelowOneMillionth(balance) {
+    return typeof balance === "string" && balance.startsWith("0.000000");
+}
+
+module.exports = {
+    truncateAmount,
+    truncateAmountNeverZero,
+    isBelowOneMillionth,
+};