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

TODO.md

@@ -45,6 +45,15 @@ but the review is broader than any of them.
 
 # Completed Steps
 
+- 2026-10-04: A token that reports more than 80 decimal places has no known
+  scale ([#350](https://git.eeqj.de/sneak/AutistMask/issues/350)). The shared
+  scale check `toDecimals()` accepted any `uint8`, but `formatUnits()` throws
+  above 80, so such a token left a swap or an ERC-20 call on the approval screen
+  undecoded, with nothing saying why. The check now stops at 80, and both
+  approval paths show the base-unit amount with the scale stated as unknown. The
+  balance list and the history list use the same check, so the same token no
+  longer stops an address's token balances from refreshing or its history from
+  loading.
 - 2026-10-04: Debug mode no longer writes RPC API keys to the console
   ([#410](https://git.eeqj.de/sneak/AutistMask/issues/410)). `debugFetch` logged
   every request's full URL and body, so an RPC endpoint with a key in its path