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/approvalVerify.test.js

@@ -28,6 +28,7 @@ const {
     TX_STAGE_NONCE,
     MAX_GAS_LIMIT,
     MAX_FEE_PER_GAS,
+    MAX_TOTAL_FEE,
 } = require("../src/shared/approvalVerify");
 const { prepareApprovalTx } = require("../src/shared/approvalTx");
 const { getSignerForAddress } = require("../src/shared/wallet");
@@ -475,18 +476,131 @@ describe("verifySignedTx field comparison", () => {
                 assertWithinCeilings({ [key]: MAX_FEE_PER_GAS + 1n }),
             ).toThrow(/fee per gas far above any plausible value/);
         }
+        // Each field at its own ceiling multiplies to about 10,000 ETH, which
+        // is exactly the combination the per-field ceilings cannot see and the
+        // product bound is for: it is refused, not accepted.
         expect(() =>
             assertWithinCeilings({
                 gasLimit: MAX_GAS_LIMIT,
                 maxFeePerGas: MAX_FEE_PER_GAS,
                 maxPriorityFeePerGas: MAX_FEE_PER_GAS,
             }),
+        ).toThrow(/network fee of up to/);
+        // An ordinary transaction — a modest gas limit and a modest fee, each
+        // far under its ceiling and their product far under the bound — passes.
+        expect(() =>
+            assertWithinCeilings({
+                gasLimit: 21000n,
+                maxFeePerGas: 2000000000n,
+                maxPriorityFeePerGas: 1000000000n,
+            }),
         ).not.toThrow();
         // Nothing to bound is not a failure: a type 2 approval carries no gas
         // price, and a bare object must not be refused for lacking one.
         expect(() => assertWithinCeilings({})).not.toThrow();
     });
 
+    // The defect this issue closes: gasLimit and maxFeePerGas each under their
+    // own ceiling, but their product — the fee a gas-consuming contract can
+    // really extract — thousands of ETH. The per-field ceilings accept it; the
+    // product bound refuses it, on either side of the screen.
+    describe("the combined fee bound", () => {
+        // A gas limit and a fee that are each comfortably under their own
+        // ceiling but multiply to well over 1 ETH: 30,000,000 gas at 100,000
+        // gwei is about 3,000 ETH.
+        const OVER = { gasLimit: 30000000n, maxFeePerGas: 100000000000000n };
+
+        test("each field is under its own ceiling", () => {
+            expect(OVER.gasLimit).toBeLessThan(MAX_GAS_LIMIT);
+            expect(OVER.maxFeePerGas).toBeLessThanOrEqual(MAX_FEE_PER_GAS);
+            expect(OVER.gasLimit * OVER.maxFeePerGas).toBeGreaterThan(
+                MAX_TOTAL_FEE,
+            );
+        });
+
+        test("assertWithinCeilings refuses the product over the bound", () => {
+            expect(() =>
+                assertWithinCeilings({
+                    ...OVER,
+                    maxPriorityFeePerGas: 1000000000n,
+                }),
+            ).toThrow(/network fee of up to 3000\.0 ETH/);
+        });
+
+        test("assertWithinCeilings bounds a legacy gasPrice the same way", () => {
+            expect(() =>
+                assertWithinCeilings({
+                    gasLimit: OVER.gasLimit,
+                    gasPrice: OVER.maxFeePerGas,
+                }),
+            ).toThrow(/network fee of up to/);
+        });
+
+        // The boundary itself, pinned rather than only some value well past
+        // it. Both fields stay under their own ceilings, so it is the product
+        // and nothing else that decides these two cases: a gas limit of 10,000
+        // at the per-gas ceiling is exactly 1 ETH.
+        test("assertWithinCeilings accepts a product exactly at the bound and refuses one wei over", () => {
+            expect(MAX_FEE_PER_GAS * 10000n).toBe(MAX_TOTAL_FEE);
+            expect(() =>
+                assertWithinCeilings({
+                    gasLimit: 10000n,
+                    maxFeePerGas: MAX_FEE_PER_GAS,
+                }),
+            ).not.toThrow();
+            expect(() =>
+                assertWithinCeilings({
+                    gasLimit: 10001n,
+                    maxFeePerGas: MAX_FEE_PER_GAS,
+                }),
+            ).toThrow(/network fee of up to/);
+        });
+
+        // The dApp path: an artifact whose fee is within each field's ceiling
+        // but over the product bound, both displayed and signed, is refused at
+        // verification just as it is at population.
+        test("verifySignedTx refuses an over-bound product even when displayed", async () => {
+            const raw = await signedWith(OVER);
+            expect(() =>
+                verifySignedTx(
+                    raw,
+                    approvedFor(TX_PARAMS, OVER),
+                    signer.address,
+                    SELECTED,
+                ),
+            ).toThrow(/network fee of up to/);
+        });
+
+        test("verifySignedTx accepts a product just under the bound", async () => {
+            // 21,000 gas at 40 gwei is 0.00084 ETH — an ordinary send.
+            const under = { gasLimit: 21000n, maxFeePerGas: 40000000000n };
+            expect(under.gasLimit * under.maxFeePerGas).toBeLessThan(
+                MAX_TOTAL_FEE,
+            );
+            const raw = await signedWith(under);
+            expect(() =>
+                verifySignedTx(
+                    raw,
+                    approvedFor(TX_PARAMS, under),
+                    signer.address,
+                    SELECTED,
+                ),
+            ).not.toThrow();
+        });
+
+        test("the refusal names the fee and the limit in a full sentence", () => {
+            try {
+                assertWithinCeilings(OVER);
+                throw new Error("expected a rejection");
+            } catch (e) {
+                expect(e.approvalMismatch).toBe(true);
+                expect(e.message).toMatch(/^[A-Z].*\.$/);
+                expect(e.message).toContain("3000.0 ETH");
+                expect(e.message).toContain("1.0 ETH");
+            }
+        });
+    });
+
     test("every field mismatch is a refusal, not a warning", async () => {
         const raw = await signedWith({ nonce: 8 });
         try {