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/approvalAmount.test.js

@@ -184,6 +184,22 @@ describe("decodeCalldata amount", () => {
         expect(line).not.toMatch(/0\.0000/);
     });
 
+    // A token added by hand carries whatever its decimals() returned, and a
+    // uint8 reaches 255, but formatUnits() throws above 80. The throw left the
+    // call undecoded rather than refused
+    // (https://git.eeqj.de/sneak/AutistMask/issues/350).
+    test("a token reporting more than 80 decimals shows base units", () => {
+        state.trackedTokens = [
+            { address: NOVEL_TOKEN, symbol: "NOVEL", decimals: 81 },
+        ];
+        expect(
+            amountLine(transferData(FIVE_THOUSAND_AT_SIX), NOVEL_TOKEN),
+        ).toBe("5000000000 base units (decimals unknown)");
+        expect(amountLine(approveData(FIVE_THOUSAND_AT_SIX), NOVEL_TOKEN)).toBe(
+            "5000000000 base units (decimals unknown)",
+        );
+    });
+
     test("an unbounded allowance is still named, with or without a scale", () => {
         expect(amountLine(approveData(MAX_UINT256), NOVEL_TOKEN)).toBe(
             "Unlimited",