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

tests/transferAmount.test.js

@@ -4,7 +4,7 @@
 // contract at signing time, with nothing comparing the two, so a token whose
 // on-chain scale differed signed an amount that was never displayed.
 
-const { parseUnits } = require("ethers");
+const { formatUnits, parseUnits } = require("ethers");
 const {
     displayedDecimals,
     transferAmountUnits,
@@ -25,7 +25,17 @@ describe("displayedDecimals", () => {
         expect(displayedDecimals(MAX_DECIMALS)).toBe(MAX_DECIMALS);
     });
 
-    test("refuses anything that is not a uint8", () => {
+    // decimals() is a uint8, but formatUnits() and parseUnits() stop at 80
+    // places, so a larger scale cannot be shown or encoded
+    // (https://git.eeqj.de/sneak/AutistMask/issues/350).
+    test("accepts exactly the scales the formatter accepts", () => {
+        expect(() => formatUnits(1n, MAX_DECIMALS)).not.toThrow();
+        expect(() => parseUnits("1", MAX_DECIMALS)).not.toThrow();
+        expect(() => formatUnits(1n, MAX_DECIMALS + 1)).toThrow();
+        expect(() => parseUnits("1", MAX_DECIMALS + 1)).toThrow();
+    });
+
+    test("refuses anything that is not a uint8 the formatter accepts", () => {
         for (const bad of [
             null,
             undefined,