harden: resolve or refuse the swap token scale instead of guessing 18 (closes #340)

tokenInfo() returned decimals 18 for any token absent from the bundled list, so the swap approval line rendered a real 1000.00 of a 6-decimal token as 0.000000000001. The scale is now resolved from what the wallet already holds (bundled list, tracked tokens, explorer-reported decimals) or refused outright, matching the rule set for the ERC-20 path in #306. A refusal reuses unknownDecimalsAmount(), so it reads as "base units (decimals unknown)" with no decimal point and no symbol, and the same string propagates to rawValue so no downstream screen can render a figure the approval screen refused. No new network call on the approval path. Verified green on all three CI contexts: check, e2e-chrome, e2e-firefox.

README.md

@@ -883,6 +883,25 @@ transaction detail view is the authoritative record and already shows exact
 precision. The 4-decimal rule is unchanged everywhere else, including for
 amounts at or above the floor on the approval screens.
 
+The floor applies only where the token's scale is known. Where it is not, the
+approval screen states base units instead of a quantity — see Unknown token
+scale below — and no truncation happens at all.
+
+**Specific Exception — unknown token scale:** Calldata carries base units and no
+scale, so every amount on the dApp approval screen needs the token's `decimals`.
+It is resolved from the bundled token list, then from the tokens the user
+tracks, then from what the block explorer reported for the contract
+(`resolveTokenDecimals()` in `src/shared/approvalAmount.js`). Where none of them
+answers, the amount is not formatted: the line reads
+`5000000000 base units (decimals unknown)`. A guessed scale is not an
+approximation but a different number — 1,000 units of a 6-decimal token
+formatted at 18 decimals reads `0.000000001` — on the screen whose only job is
+to state what is being authorized. Both amount paths of that screen take this
+rule: the ERC-20 `transfer`/`approve` line (`src/popup/views/approval.js`) and
+the swap's `Amount` and `Min. received` lines (`src/shared/uniswap.js`). An
+unbounded allowance or permit needs no scale to describe and is still shown as
+`Unlimited`.
+
 #### Partial USD totals
 
 Prices are fetched for the top 25 tokens only, so an address can hold assets the