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/uniswapUnknownDecimals.test.js

@@ -126,6 +126,19 @@ describe("a swap of a token outside the bundled list", () => {
     test("a bundled token on the other side still formats", () => {
         expect(swapDetail(data(), "Min. received").value).toBe("0.5000 WETH");
     });
+
+    // A token added by hand carries whatever its decimals() returned, and a
+    // uint8 reaches 255, but formatUnits() throws above 80. The throw left the
+    // whole swap undecoded rather than refused
+    // (https://git.eeqj.de/sneak/AutistMask/issues/350).
+    test("refuses to format when the token reports more than 80 decimals", () => {
+        state.trackedTokens = [
+            { address: NOVEL, symbol: "NOVEL", decimals: 81 },
+        ];
+        expect(swapDetail(data(), "Amount").value).toBe(
+            "1000000000 base units (decimals unknown)",
+        );
+    });
 });
 
 describe("the Min. received line takes the same rule", () => {