harden: bound the total network fee by gasLimit × fee, on both send paths (closes #399)

The two per-field ceilings in approvalVerify.js were checked independently,
but the fee a validator is paid is gasLimit × fee per gas: a gas limit and a
fee each under their own ceiling still multiply to thousands of ETH, which a
gas-consuming contract really collects. assertWithinCeilings now also bounds
that product against MAX_TOTAL_FEE (1 ETH), so both callers — populating the
dApp transaction and verifying the signed artifact — refuse it with a full
sentence naming the fee and the limit.

The wallet's own send in confirmTx.js pinned no fee fields, so ethers filled
them from the node with no bound; it now populates the transaction and runs the
same check before signing, showing the same error in the confirmation screen's
reserved errors box so nothing on screen moves.

Model: opus-4-8

tests/approvalTx.test.js

@@ -212,6 +212,24 @@ describe("prepareApprovalTx", () => {
         ).rejects.toThrow(/gas limit no network this wallet supports/);
     });
 
+    // The combined bound at population: a gas limit and a fee that are each
+    // under their own ceiling but multiply to thousands of ETH is refused
+    // before the approval window opens, so the user is never shown a
+    // balance-draining fee to click past.
+    test("refuses a fee whose product with the gas limit is over the bound", async () => {
+        const gouging = providerWith({
+            estimateGas: async () => 30000000n,
+            getFeeData: async () => ({
+                gasPrice: MAX_FEE_PER_GAS,
+                maxFeePerGas: MAX_FEE_PER_GAS,
+                maxPriorityFeePerGas: 1000000000n,
+            }),
+        });
+        await expect(
+            prepareApprovalTx(gouging, signer.address, TX_PARAMS),
+        ).rejects.toThrow(/network fee of up to/);
+    });
+
     // No approval and no window: the failure goes back to the page the click
     // came from, in a sentence.
     test("reports a failed estimate as a full sentence", async () => {