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 "+"), and
getSignerForAddress() derives at that index and refuses any key whose
address differs from the record's. The export screen reports that
refusal instead of calling the password wrong.

Model: opus-5-5

tests/approvalNonce.test.js

@@ -136,7 +136,9 @@ async function openTxApproval(approvedTx) {
             xpub: "xpub-wallet-1",
             encryptedSecret: "encrypted-secret-1",
             nextIndex: 1,
-            addresses: [{ address: FROM, balance: "0", tokenBalances: [] }],
+            addresses: [
+                { address: FROM, index: 0, balance: "0", tokenBalances: [] },
+            ],
         },
     ];
     approval.init({});