fix: show the true token amount on the approval screen, or none at all (closes #306)

decodeCalldata resolved an ERC-20 amount's scale from the 512-entry bundled
token list alone and fell back to 18 decimals for everything else. Most tokens
are outside that list, including anything the user added by contract address, so
a transfer of 5000000000 units of a 6-decimal token - 5,000 tokens - was drawn as
"Amount 0.0000". A user who reads zero confirms, and loses the balance or grants
the allowance.

The new src/shared/approvalAmount.js resolves decimals from the bundled list,
then state.trackedTokens, then the decimals the block explorer already reported
in addr.tokenBalances, accepting only a real uint8 from any of them and refusing
a scale the explorer's own entries disagree about.

Where no source knows the scale the amount is not formatted at all: the line
reads "5000000000 base units (decimals unknown)". A formatted number computed
from a guessed scale is the defect itself, and for a token with fewer decimals
than the guess it is wrong in the direction that reads as zero. approve is
covered alongside transfer; an unbounded allowance still reads "Unlimited",
which needs no scale. The same string is carried to the status screens, so no
formatted figure reappears downstream.

Verified failing first: restoring only the old lookup fails 6 of the 15 new
tests, with the unknown-decimals transfer case receiving exactly "0.0000".

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)).