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

Max fills in a token's balance, cut down to the 18 decimal places the
confirmation screen accepts, or for ETH the exact balance minus the fee
reserve the confirmation screen's balance check gates on. An ETH fee estimate
that finishes after the Send screen was left, or its address, holding,
recipient or amount changed, fills nothing in. 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

README.md

@@ -1467,18 +1467,17 @@ view would leave a wallet one click from deletion.
     - Amount input with current balance display, which reads
       `Current balance: unknown (SYMBOL)` for a token whose scale is unknown, as
       ConfirmTx's balance line does (see Unknown token scale)
-    - "Max" button beside the amount input, always in place. It fills in the
-      most the selected holding can send: a token's whole balance, cut down to
-      the 18 decimal places ConfirmTx accepts for a token that has more, or for
-      ETH the exact balance minus the network fee reserve that ConfirmTx's
-      balance check gates on, never the rounded balance shown above it. The ETH
-      fee is estimated for the recipient entered, so it asks for a recipient
-      first; an estimate that finishes after the screen was left or the address,
-      holding or recipient changed fills nothing in. Where there is nothing to
-      fill in, a flash message says why: the balance does not cover the fee, the
-      fee could not be estimated, or the token's balance is unknown or zero.
-      Typing in the amount makes it an ordinary amount; changing what to send
-      clears an amount Max filled in
+    - "Max" button beside the amount input, always in place. It fills in a
+      token's balance, cut down to the 18 decimal places ConfirmTx accepts for a
+      token that has more, or for ETH the exact balance minus the network fee
+      reserve that ConfirmTx's balance check gates on, never the rounded balance
+      shown above it. The ETH fee is estimated for the recipient entered, so it
+      asks for a recipient first; an estimate that finishes after the screen was
+      left or the address, holding or recipient changed fills nothing in. Where
+      there is nothing to fill in, a flash message says why: the balance does
+      not cover the fee, the fee could not be estimated, or the token's balance
+      is unknown or zero. Typing in the amount makes it an ordinary amount;
+      changing what to send clears an amount Max filled in
     - "Review" button, disabled until the recipient validates
 - **Transitions**:
     - "Review" (valid inputs, ENS resolved) → **ConfirmTx**

docs/README.md

@@ -240,8 +240,9 @@ screen. Tokens can also be added from Settings, under "Tracked Tokens".
 2. Select what to send (ETH, or any ERC-20 token with a balance on this address
    that survives the spam filters).
 3. Enter the recipient address or ENS name (e.g. `vitalik.eth`).
-4. Enter the amount, or click "Max" to fill in everything you can send: a
-   token's whole balance, or your ETH balance minus the network fee.
+4. Enter the amount, or click "Max" to fill it in: a token's balance, cut to 18
+   decimal places, or your ETH balance minus the amount reserved for the network
+   fee.
 5. Click "Review" to see the confirmation screen.
 
 The confirmation screen shows:

tests/sendMax.test.js

@@ -419,6 +419,34 @@ describe("Max on an ETH send", () => {
         expect(text("flash-msg")).toBe("");
     });
 
+    test("fills nothing in when the holding was changed while the fee was estimated", async () => {
+        await refreshWith(BALANCE_WEI, [tokenRow(10n ** 18n)]);
+        // Opened from the home screen, where the dropdown picks the holding.
+        openSend(null);
+        el("send-token").value = "ETH";
+        const release = holdFeeEstimate();
+        const pressed = pressMax();
+        el("send-token").value = TOKEN;
+        el("send-token").handlers.get("change")();
+        release();
+        await pressed;
+        expect(el("send-amount").value).toBe("");
+        expect(text("flash-msg")).toBe("");
+    });
+
+    test("keeps an amount typed while the fee was estimated", async () => {
+        await refreshWith(BALANCE_WEI);
+        openSend();
+        const release = holdFeeEstimate();
+        const pressed = pressMax();
+        el("send-amount").value = "0.5";
+        el("send-amount").handlers.get("input")();
+        release();
+        await pressed;
+        expect(el("send-amount").value).toBe("0.5");
+        expect(text("flash-msg")).toBe("");
+    });
+
     test("fills in once the held fee estimate arrives with nothing changed", async () => {
         await refreshWith(BALANCE_WEI);
         openSend();