feat: remove an address from an HD wallet, behind a confirmation (closes #162)

Address rows on Home now carry an [x] control on wallets that derive their
addresses from an extended key and hold more than one; it opens a confirmation
screen before anything is removed.

Removing an address destroys nothing: it stays derivable from key material the
wallet still holds, and any funds at it stay where they are. That is also why
the screen is not password-gated, unlike delete-wallet — a password gates the
disclosure or destruction of a secret, and this does neither.

Getting the address back into the list is another matter, and the copy states
it exactly rather than promising a route the app refuses. "+" derives the next
unused index, because the derivation counter is a high-water mark and is not
rewound, and re-importing the wallet's key material is rejected as a duplicate
for as long as the wallet is present — which it always is here, since a wallet
never gives up its last address. What works is deleting the whole wallet in
Settings, which asks for the password and destroys the stored secret, then
importing again: the scan that follows rediscovers the address only if it has
on-chain activity, and an address that was never used does not come back at
all. The text is built by recoveryPathText() rather than sitting in index.html
so it can name the wallet's own kind of key material, an xprv wallet having no
recovery phrase to re-import.

A balance is surfaced as a warning, never a refusal, and holding something
means any ERC-20 as well as ETH, at any size: an address with no ETH and a
stablecoin position must not get a blank line on the screen whose job is to
warn. The warning names no figure of its own, because the balance lines round
to four decimals and a sentence built from a rounded number would report
0.0000 ETH for an address holding real money; the amounts come from the same
balanceLinesForAddress() and getAddressValueUsd() every other screen uses.

The state transition lives next to the wallet one in
src/shared/walletDelete.js and shares its address comparison, site-permission
cleanup and broadcast, so the rules match one level down: the last address of
a wallet is never removable, the selection moves only when it was the address
removed, an index after the splice is decremented, a selection in another
wallet is untouched, and AUTISTMASK_ACTIVE_CHANGED is broadcast when the
active address moves so a connected site stops being told about an address the
user removed.

src/shared/walletDelete.js

@@ -1,5 +1,22 @@
-// Wallet deletion state transition, kept out of the view so the selection
-// and broadcast rules are testable without a DOM.
+// Wallet and address deletion state transitions, kept out of the views so the
+// selection and broadcast rules are testable without a DOM.
+
+// Two records of the same address can be stored in different cases, so
+// address equality is never a literal string comparison.
+function sameAddress(a, b) {
+    if (a === null || a === undefined || b === null || b === undefined) {
+        return false;
+    }
+    return String(a).toLowerCase() === String(b).toLowerCase();
+}
+
+// Forget every site permission held against the given addresses.
+function dropSitePermissions(state, addresses) {
+    for (const addr of addresses) {
+        delete state.allowedSites[addr];
+        delete state.deniedSites[addr];
+    }
+}
 
 // Remove wallet `walletIdx` from `state` and repair the derived state.
 //
@@ -18,19 +35,13 @@ function removeWalletFromState(state, walletIdx) {
     const wallet = state.wallets[walletIdx];
     const addresses = (wallet.addresses || []).map((a) => a.address);
     const previousActive = state.activeAddress;
-    const activeWasDeleted =
-        previousActive !== null &&
-        previousActive !== undefined &&
-        addresses.some(
-            (a) => a.toLowerCase() === String(previousActive).toLowerCase(),
-        );
+    const activeWasDeleted = addresses.some((a) =>
+        sameAddress(a, previousActive),
+    );
 
     state.wallets.splice(walletIdx, 1);
 
-    for (const addr of addresses) {
-        delete state.allowedSites[addr];
-        delete state.deniedSites[addr];
-    }
+    dropSitePermissions(state, addresses);
 
     state.hasWallet = state.wallets.length > 0;
 
@@ -58,6 +69,77 @@ function removeWalletFromState(state, walletIdx) {
     return { activeAddressChanged: state.activeAddress !== previousActive };
 }
 
+// Whether a wallet may be offered a per-address remove control, and the same
+// gate the removal itself is held behind.
+//
+// Only a wallet that derives its addresses from an extended key can hold more
+// than one, so only those get the control — a key wallet has exactly one
+// address and no "+" button either. The last address of any wallet is never
+// removable: a wallet with no addresses is what delete-wallet is for.
+function canRemoveAddress(wallet) {
+    if (!wallet) return false;
+    if (wallet.type !== "hd" && wallet.type !== "xprv") return false;
+    return (wallet.addresses || []).length > 1;
+}
+
+// Remove address `addrIdx` of wallet `walletIdx` and repair the derived state.
+//
+// Nothing is destroyed here. The address stays derivable from the wallet's own
+// key material and any funds at it are untouched; this only stops the wallet
+// tracking it. `nextIndex` is deliberately left alone — it is a derivation
+// high-water mark, so "+" derives a fresh index rather than handing back the
+// address just removed, and the gap it leaves is within what
+// `scanForAddresses()` re-discovers on a later import.
+//
+// The rules mirror removeWalletFromState() one level down:
+//   - The call is refused unless canRemoveAddress() allows it, so the last
+//     address of a wallet always survives.
+//   - Site permissions are dropped for the removed address.
+//   - `selectedAddress` follows the splice, but only within the wallet that
+//     lost the address: it is decremented when an earlier address was
+//     removed, and falls back to that wallet's first address when the
+//     selection itself was removed. `selectedWallet` never moves, because the
+//     wallet list does not.
+//   - `activeAddress` moves only when it was the removed address, and then to
+//     the wallet's first remaining address.
+//
+// Returns whether the address was removed and whether `activeAddress`
+// changed, so the caller can broadcast it.
+function removeAddressFromState(state, walletIdx, addrIdx) {
+    const wallet = state.wallets[walletIdx];
+    const refused = { removed: false, activeAddressChanged: false };
+    if (!canRemoveAddress(wallet)) return refused;
+    if (!wallet.addresses[addrIdx]) return refused;
+
+    const address = wallet.addresses[addrIdx].address;
+    const previousActive = state.activeAddress;
+    const activeWasRemoved = sameAddress(address, previousActive);
+
+    wallet.addresses.splice(addrIdx, 1);
+
+    dropSitePermissions(state, [address]);
+
+    if (state.selectedWallet === walletIdx) {
+        if (state.selectedAddress === addrIdx) {
+            state.selectedAddress = 0;
+        } else if (
+            typeof state.selectedAddress === "number" &&
+            state.selectedAddress > addrIdx
+        ) {
+            state.selectedAddress -= 1;
+        }
+    }
+
+    if (activeWasRemoved) {
+        state.activeAddress = wallet.addresses[0].address;
+    }
+
+    return {
+        removed: true,
+        activeAddressChanged: state.activeAddress !== previousActive,
+    };
+}
+
 // Tell the background the active address changed, so it re-emits
 // accountsChanged to connected sites. Same call shape as the address
 // switch in the home view.
@@ -67,4 +149,9 @@ function broadcastActiveChanged() {
     runtime.sendMessage({ type: "AUTISTMASK_ACTIVE_CHANGED" });
 }
 
-module.exports = { removeWalletFromState, broadcastActiveChanged };
+module.exports = {
+    canRemoveAddress,
+    removeAddressFromState,
+    removeWalletFromState,
+    broadcastActiveChanged,
+};