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().

Model: opus-5-5

src/popup/views/send.js

@@ -24,7 +24,7 @@ const { resolveSymbol } = require("../../shared/tokenList");
 const { isLowHolderCount } = require("../../shared/holders");
 const { isSpoofedSymbol } = require("../../shared/symbolSpoof");
 const {
-    truncateAmountNeverZero,
+    truncateAmount,
     isBelowOneMillionth,
 } = require("../../shared/amountDisplay");
 const {
@@ -239,7 +239,7 @@ function updateSendBalance() {
             addr.balance == null
                 ? "Current balance: unknown (" + nativeCurrency() + ")"
                 : "Current balance: " +
-                  truncateAmountNeverZero(addr.balance) +
+                  truncateAmount(addr.balance) +
                   " " +
                   nativeCurrency();
     } else {
@@ -258,18 +258,15 @@ function updateSendBalance() {
         $("send-balance").textContent =
             bal == null
                 ? "Current balance: unknown (" + symbol + ")"
-                : "Current balance: " +
-                  truncateAmountNeverZero(bal) +
-                  " " +
-                  symbol;
+                : "Current balance: " + truncateAmount(bal) + " " + symbol;
     }
 }
 
 // Fill the amount field with the most the selected holding can send: a
 // token's whole balance (cut to 18 decimal places), or for ETH the exact
 // balance minus the fee reserve the confirmation screen checks against, never
-// the rounded balance the screen shows. Where there is nothing to fill in, a
-// flash message says why.
+// the balance the screen shows, which is cut to four decimals. Where there is
+// nothing to fill in, a flash message says why.
 async function fillMaxAmount() {
     const addr = currentAddress();
     if (!addr) return;