feat: warn when sending to address with zero tx history (#82)

On the confirm-tx screen, asynchronously check the recipient address
via Blockscout API. If the address has never sent or received any
transactions, display a prominent red warning banner.

Closes #82

src/shared/transactions.js

@@ -251,4 +251,36 @@ function filterTransactions(txs, filters = {}) {
     return { transactions: filtered, newFraudContracts: newFraud };
 }
 
-module.exports = { fetchRecentTransactions, filterTransactions };
+async function hasTransactionHistory(address, blockscoutUrl) {
+    try {
+        const resp = await debugFetch(blockscoutUrl + "/addresses/" + address);
+        if (!resp.ok) {
+            // If Blockscout returns 404, the address has never been seen on-chain.
+            if (resp.status === 404) return false;
+            log.errorf(
+                "blockscout address check:",
+                resp.status,
+                resp.statusText,
+            );
+            return null; // unknown
+        }
+        const data = await resp.json();
+        // Blockscout v2 address endpoint returns tx counts.
+        // An address with no history may still exist (e.g. received ETH once
+        // but shows 0 outgoing). We check both transactions_count and
+        // token_transfers_count to be thorough.
+        const txCount =
+            (parseInt(data.transactions_count, 10) || 0) +
+            (parseInt(data.token_transfers_count, 10) || 0);
+        return txCount > 0;
+    } catch (e) {
+        log.errorf("hasTransactionHistory error:", e.message);
+        return null; // unknown, don't block the user
+    }
+}
+
+module.exports = {
+    fetchRecentTransactions,
+    filterTransactions,
+    hasTransactionHistory,
+};