harden: verify all approval fields and make failed signing retryable (closes #174)

verifySignedTx compared only from, to, value and data, so a signed
transaction could differ from the approval in chain id, nonce, gas limit
or any fee field and still be broadcast. Worse, it named the fields it
checked and so admitted every field it did not name: a type 4 artifact
carrying an EIP-7702 authorization passed verification, paying the
approved amount to the approved recipient and, in the same transaction,
permanently installing another contract's code at the signer's own
account.

The check is now an allowlist in both directions. The transaction type
must be 0, 1 or 2 — the only types this wallet signs — so no later
EIP-2718 type can bring a field along; authorizationList, blobs, blob
commitments and blob gas fees are refused by name; and the access list
is compared with the approval. Every consequential field is compared and
any mismatch refuses outright: the chain id against the selected network
(and against the approval when the page fixed one), plus nonce, gas
limit, gasPrice, maxFeePerGas and maxPriorityFeePerGas wherever the
approval carries a value, together with the fee mechanism the approval
implies. Fields the approval does not carry are populated locally by the
popup and have no approved value to compare against, so they are held to
absolute ceilings instead. Verification then closes by rebuilding the
transaction from exactly those checked fields and comparing the unsigned
bytes, so an artifact carrying anything this module does not account for
is refused without having to be named first.

An approved value that is not a number now refuses like every other
quantity rather than escaping as a raw BigInt conversion error, which
was reported as retryable and left a live button that could never
succeed.

A failed signing attempt also left a button that could not succeed: the
background deleted the approval before it broadcast, so a retry found
nothing to sign. The approval is now retired once the request has an
outcome, and the background tells the popup which stage failed. A popup
that could not sign is retryable; a mismatch spends the approval; a
failed broadcast is terminal, because the node may have accepted the
transaction and still failed to answer and the popup's retry re-signs at
a freshly fetched nonce rather than re-broadcasting the same bytes,
which would send the approved transfer twice.

Keeping the approval alive for that retry cost it its single use: the
handler read it, then verified and broadcast asynchronously, so a second
AUTISTMASK_TX_RESPONSE carrying the same id started an independent
verify and broadcast instead of finding nothing. With the ordinary dApp
approval shape the page fixes no nonce, so two artifacts signed at
different nonces both verify and the approved transfer goes out twice; a
reloaded approval window during a slow broadcast is enough to send it,
since the only guard was popup-local button state. The approval is now
claimed synchronously, before the first await, and released only when an
attempt fails in a way the user may retry. Same interlock on
AUTISTMASK_SIGN_RESPONSE.

Verification also compared only the decode against itself: both sides of
the closing byte comparison derive from one Transaction.from(), while
what is broadcast is the artifact string. An artifact re-encoded with a
leading zero byte on an RLP quantity therefore decoded to the approved
transaction, passed, and broadcast different bytes. The artifact is now
required to be the canonical encoding of its own decode, which is what
makes the claim that it *is* the approved transaction true.

The background's approval wiring had no tests, which is where both
defects lived. It has them now, driven through the real message listener
from eth_sendTransaction to broadcast.

TODO.md

@@ -51,6 +51,13 @@ undefined identifiers, which is how
   Ethereum mainnet ERC-20s — with `TOKENS` in `src/shared/tokenList.js` named as
   the authoritative set
   ([#239](https://git.eeqj.de/sneak/AutistMask/issues/239)).
+- 2026-08-11: Approval verification became an allowlist — transaction type
+  restricted to 0/1/2 so an EIP-7702 delegation can no longer ride along on an
+  approved transfer, every consequential field compared, the artifact
+  re-serialized from the checked fields alone and its exact bytes required to be
+  the canonical encoding of what was broadcast, and one approval now yields at
+  most one broadcast; broadcast failure is terminal
+  ([#174](https://git.eeqj.de/sneak/AutistMask/issues/174)).
 - 2026-08-11: Known-symbol spoof verification became a Settings toggle
   (`hideSpoofedSymbols`), on by default, governing the transaction-history
   filter and the fraud-contract learning it feeds