fix: estimate a request even when the site sends its gas limit (closes #554)

ethers asks the node for a gas estimate only when the request has no gas
limit, and that estimate is the wallet's only check that a transaction
does not revert. Since a site's `gas` became the gas limit, a reverting
request carrying one opened the approval window and could be signed and
mined to fail. Such a request is now estimated first, with the site's
limit and under the same timeout; a failure goes back to the site with no
window, and the site's gas limit is still the one shown and signed. A
limit too low for the call now fails the same way.

Model: opus-5-5

src/shared/approvalTx.js

@@ -201,6 +201,7 @@ async function prepareApprovalTx(provider, from, txParams) {
         throw fail("There is no active address to send this transaction from.");
     }
     const request = requestFrom(txParams || {}, from);
+    const signer = new VoidSigner(getAddress(from), provider);
 
     let populated;
     try {
@@ -208,10 +209,20 @@ async function prepareApprovalTx(provider, from, txParams) {
         // nonce, gas, fee and chain id are populated exactly as they were when
         // the popup did this. VoidSigner cannot sign, which is the point: the
         // background prepares, the popup signs.
+        //
+        // ethers asks the node for a gas estimate only when the request has
+        // no gas limit, and that estimate is the only check that the
+        // transaction does not revert. So a request carrying the site's gas
+        // limit is estimated first, with that limit: one that reverts, or
+        // needs more gas than the site gave it, fails here as well. The
+        // site's gas limit is still the one populated.
         populated = await withTimeout(
-            new VoidSigner(getAddress(from), provider).populateTransaction(
-                request,
-            ),
+            (async () => {
+                if (present(request.gasLimit)) {
+                    await signer.estimateGas(request);
+                }
+                return signer.populateTransaction(request);
+            })(),
             POPULATE_TIMEOUT_MS,
             "The transaction could not be prepared: the network did not answer in time.",
         );