fix: the balance lists and Home cut amounts 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. They 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 screens that state what is authorized keep the floor
of truncateAmountNeverZero().

Model: opus-5-5

README.md

@@ -1191,8 +1191,8 @@ These helpers write one kind of value on every screen that shows it:
 
 Most screens show token and ETH amounts with four decimal places. History rows
 cut amounts to four decimals and never round up (`truncateAmount()` in
-`src/shared/amountDisplay.js`). The balance lists and Home's ETH balance round
-to four places. The screens that state what is being authorized cut as the
+`src/shared/amountDisplay.js`). The balance lists and Home's ETH balance cut
+them the same way. The screens that state what is being authorized cut as the
 history does, and extend an amount below 0.0001 to its first nonzero digit
 (`truncateAmountNeverZero()`). So a nonzero amount never reads as zero there.
 Those screens are: