harden: a token scale above 80 decimal places is refused as unknown (closes #350)

toDecimals() accepted any uint8 scale, but formatUnits() and parseUnits()
refuse more than 80 decimal places. A token reporting 81 to 255 made the
formatter throw, and the catch in the swap decoder and in the ERC-20 decoder
turned that into an undecoded approval screen with nothing saying why.

MAX_DECIMALS is now 80, the formatter's own limit, so such a scale is
treated exactly like an unknown one: both approval paths show the base-unit
amount with the scale stated as unknown. The balance list, the history list
and the Send screen use the same check.

Model: opus-5-5

src/shared/transferAmount.js

@@ -27,9 +27,11 @@
 
 const { parseUnits } = require("ethers");
 
-// Solidity's decimals() returns a uint8, so anything outside that range is not
-// an answer this wallet can use.
-const MAX_DECIMALS = 255;
+// Solidity's decimals() returns a uint8, but ethers' formatUnits() and
+// parseUnits() refuse more than 80 decimal places ("invalid FixedNumber
+// decimals (too large)"). A scale of 81 to 255 can be neither displayed nor
+// encoded, so it is not an answer this wallet can use, the same as no answer.
+const MAX_DECIMALS = 80;
 
 const UNKNOWN_DISPLAYED_DECIMALS_MESSAGE =
     "The transfer was not sent, because the number of decimal places this" +
@@ -55,7 +57,7 @@ function mismatchMessage(displayed, onChain) {
 // A decimals value from any source as a number, or null if it is not one.
 // decimals() comes back from ethers as a bigint and the explorer's copy arrives
 // as a string, so both of those are accepted alongside a plain number; anything
-// fractional, negative, out of uint8 range, or of any other type at all is not.
+// fractional, negative, above MAX_DECIMALS, or of any other type at all is not.
 //
 // The types are enumerated rather than coerced because Number() is far too
 // willing: Number([]) is 0 and Number(true) is 1, so a coercing check would