feat: a "Max" button on the Send screen (closes #198)

Max fills in a token's whole balance, or for ETH the exact balance minus
the fee reserve the confirmation screen's balance check gates on. The
confirmation screen works a max ETH amount out again from its own fee
estimate and signs it with that estimate's fee fields, so a fee that rose
before signing cannot push amount plus fee above the balance.
validateTransfer() still gates every send, the check that ETH covers a
token send's fee included. Where there is nothing to fill in, a flash
message says why.

Model: opus-5-5

tests/txValidation.test.js

@@ -6,6 +6,7 @@ const {
     FEE_UNAVAILABLE,
     feeReserveWei,
     feeEstimateWei,
+    maxEthAmount,
     toFixedPoint,
     validateTransfer,
 } = require("../src/shared/txValidation");
@@ -306,6 +307,55 @@ describe("feeEstimateWei", () => {
     });
 });
 
+// The amount the Send screen's Max fills in for ETH: the exact balance, as
+// balances.js stores it, minus the fee reserve. Never the four-decimal balance
+// the Send screen shows.
+describe("maxEthAmount", () => {
+    // The Send screen shows this balance as 1.2345.
+    const BALANCE = "1.234567890123456789";
+
+    test("is the exact balance minus the fee, to the wei", () => {
+        expect(maxEthAmount(BALANCE, FEE)).toBe("1.234147890123456789");
+        expect(
+            parseEther(BALANCE) - parseEther(maxEthAmount(BALANCE, FEE)),
+        ).toBe(FEE);
+    });
+
+    test("passes validateTransfer with exactly the fee left behind", () => {
+        const r = validateTransfer({
+            isErc20: false,
+            amount: maxEthAmount(BALANCE, FEE),
+            ethBalance: BALANCE,
+            feeStatus: FEE_KNOWN,
+            feeWei: FEE,
+        });
+        expect(r).toEqual({ canSend: true, codes: [] });
+    });
+
+    test("is one wei when the balance is one wei more than the fee", () => {
+        expect(maxEthAmount("0.000420000000000001", FEE)).toBe(
+            "0.000000000000000001",
+        );
+    });
+
+    test("is null when the balance does not cover the fee", () => {
+        expect(maxEthAmount("0.0001", FEE)).toBe(null);
+        expect(maxEthAmount("0.0", FEE)).toBe(null);
+    });
+
+    test("is null when the balance covers the fee and nothing more", () => {
+        expect(maxEthAmount("0.00042", FEE)).toBe(null);
+    });
+
+    test("is null on a balance or fee it cannot do exact arithmetic on", () => {
+        expect(maxEthAmount(undefined, FEE)).toBe(null);
+        expect(maxEthAmount("not a number", FEE)).toBe(null);
+        expect(maxEthAmount(BALANCE, null)).toBe(null);
+        expect(maxEthAmount(BALANCE, -1n)).toBe(null);
+        expect(maxEthAmount(BALANCE, 420000000000000)).toBe(null);
+    });
+});
+
 // Everything that is not a usable fee blocks exactly as FEE_UNAVAILABLE does.
 // Each of these previously returned { canSend: true, codes: [] } — counting no
 // fee at all, on a full-balance send, in the direction that lets money out.