fix: every balance is cut to four decimals (closes #757)

The balance lists and Home's ETH line rounded with toFixed(4), so 0.99996
ETH read 1.0000 there and 0.9999 on Send. Send's current balance and
ConfirmTx's balance line kept the nonzero floor, so 0.00004 USDC read
0.0000 in the lists and 0.00004 on Send. All four now cut the stored
decimal string with truncateAmount(), as the history rows do.
balanceLine() takes the stored balance string instead of a parsed number,
and escapes the quantity cell. The amounts, fees and allowances a screen
asks the user to approve keep the floor of truncateAmountNeverZero().

Disclosure: for a holding below 0.0001, ConfirmTx shows the amount sent
beside a balance that reads 0.0000.

Model: opus-5-5

src/popup/views/addressToken.js

@@ -159,10 +159,10 @@ function renderBalance() {
     const addr =
         state.wallets[state.selectedWallet].addresses[state.selectedAddress];
     const tokenId = state.selectedToken;
-    let amount, price;
+    let balance, price;
     if (tokenId === "ETH") {
         // null when no refresh has read the balance.
-        amount = unknownableAmount(addr.balance);
+        balance = addr.balance;
         price = getPrice("ETH");
     } else {
         const id = tokenId.toLowerCase();
@@ -175,12 +175,13 @@ function renderBalance() {
         // null when the scale is unknown, or when no refresh has read the
         // token's balance: no quantity to show, and none to price.
         // balanceLine() states that rather than printing 0.0000.
-        if (tb) amount = unknownableAmount(tb.balance);
-        else amount = tracked && tracked.balanceNotFetched ? null : 0;
+        if (tb) balance = tb.balance;
+        else balance = tracked && tracked.balanceNotFetched ? null : "0";
         price = getPrice(tokenId);
     }
 
     // USD total for this token only
+    const amount = unknownableAmount(balance);
     const usdVal = price && amount !== null ? amount * price : null;
     const usdStr = formatUsd(usdVal);
     $("address-token-usd-total").innerHTML = escapeHtml(usdStr) || " ";
@@ -188,7 +189,7 @@ function renderBalance() {
     // Single token balance line (no tokenId — not clickable here)
     $("address-token-balance").innerHTML = balanceLine(
         currentSymbol,
-        amount,
+        balance,
         price,
     );
 }