fix: resolve approval-screen token decimals, and refuse to format an unknown scale (closes #306)

decodeCalldata consulted only the 512-entry bundled list and defaulted to 18
decimals, so a transfer of 5,000 units of a 6-decimal token rendered
"Amount 0.0000" and the user confirmed a drain reading zero. The same
understatement applied to approve, where an unbounded allowance also rendered
0.0000.

Decimals now resolve from the bundled list, then trackedTokens, then the
address's explorer-reported entry, with uint8 validation and a refusal when
sources for one contract disagree. When no source knows the scale, no
formatUnits call is reached at all: the line renders raw base units with an
explicit "decimals unknown" warning, and the same string reaches
pendingTxDetails.amount so the status screens carry no formatted figure either.

Verified failing first two independent ways: restoring the old
`token ? token.decimals : 18` fails 6 of 15 new tests with the unknown case
reporting "0.0000"; making the resolver return 18 rather than null on the
unknown path fails a different 6, spanning resolver and render levels.

TODO.md

@@ -44,6 +44,20 @@ but the review is broader than any of them.
 
 # Completed Steps
 
+- 2026-08-20: The dApp approval screen no longer shows a token transfer it
+  cannot scale as `0.0000`
+  ([#306](https://git.eeqj.de/sneak/AutistMask/issues/306)). `decodeCalldata`
+  read decimals from the 512-entry bundled token list alone and fell back to 18,
+  so every token outside it — most of them, including anything the user added by
+  contract address — was displayed at the wrong scale: a `transfer` of 5,000
+  units of a 6-decimal token read as `0.0000`, and a user who reads zero
+  confirms the drain. The new `src/shared/approvalAmount.js` resolves the scale
+  from the bundled list, then `state.trackedTokens`, then the decimals the block
+  explorer already reported in `addr.tokenBalances`, and refuses one the
+  explorer's own entries disagree about. Where no source knows it, the amount
+  line is not formatted at all: it shows the base-unit integer and states that
+  the scale is unknown, for `approve` as well as `transfer`. An unbounded
+  allowance still reads `Unlimited`, which needs no scale.
 - 2026-08-20: A web page can no longer switch the wallet's chain, and switching
   no longer destroys the user's endpoints
   ([#308](https://git.eeqj.de/sneak/AutistMask/issues/308)).