fix: sign and export with each address's own derivation index (closes #510)

Sending, site transactions, site signatures and private key export
derived the key at the address's position in the wallet's list. After
an import whose scan passed over an unused index, or after removing an
address that was not the last, that position named another address's
key.

Each HD and xprv address record now stores the index it was derived at
(at wallet creation, by the import scan, by "+"). getSignerForAddress()
derives at that index, refuses a record without one, and refuses any key
whose address differs from the record's. The confirmation screen signs
as the From address it shows, not the selected one. The export screen
reports a refusal instead of calling the password wrong.

Model: opus-5-5

src/popup/views/exportPrivkey.js

@@ -135,7 +135,18 @@ async function reveal() {
         // decrypt ran, the wipe has already happened, so the key is not
         // even derived, let alone written.
         if (!isCurrentReveal(generation)) return;
-        const signer = getSignerForAddress(wallet, addressIndex, secret);
+        let signer;
+        try {
+            signer = getSignerForAddress(
+                wallet,
+                wallet.addresses[addressIndex],
+                secret,
+            );
+        } catch (e) {
+            // The password was right; it is the key that was refused.
+            showError("export-privkey-password-error", e.message);
+            return;
+        }
         $("export-privkey-password").value = "";
         $("export-privkey-password-section").classList.add("hidden");
         $("export-privkey-value").textContent = signer.privateKey;