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

README.md

@@ -926,7 +926,10 @@ rule: the ERC-20 `transfer`/`approve` line (`src/popup/views/approval.js`) and
 the swap's `Amount` and `Min. received` lines (`src/shared/uniswap.js`). The
 token permission warning on the signature screen takes the same rule for its
 amounts. An unbounded allowance or permit needs no scale to describe and is
-still shown as `Unlimited`.
+still shown as `Unlimited`. A source's answer counts only if it is a whole
+number from 0 to 80: `decimals()` returns a `uint8`, but `formatUnits()` cannot
+format more than 80 decimal places, so a token that reports 81 to 255 is shown
+as one whose scale nothing knows.
 
 The rule holds only if nothing invents a scale UPSTREAM of it. Those three
 sources are read as authoritative, so a value written into one of them cannot be