feat: show red warning when sending to address with zero tx history

On the confirm-tx view, asynchronously check the recipient address
transaction count via getTransactionCount(). If zero, display a
prominent red warning advising the user to double-check the address.

Closes #82

src/popup/views/confirmTx.js

@@ -244,6 +244,7 @@ function show(txInfo) {
     showView("confirm-tx");
 
     estimateGas(txInfo);
+    checkRecipientHistory(txInfo);
 }
 
 async function estimateGas(txInfo) {
@@ -286,6 +287,28 @@ async function estimateGas(txInfo) {
     }
 }
 
+async function checkRecipientHistory(txInfo) {
+    try {
+        const provider = getProvider(state.rpcUrl);
+        const txCount = await provider.getTransactionCount(txInfo.to);
+        if (txCount === 0) {
+            const warningsEl = $("confirm-warnings");
+            const warning =
+                `<div class="border border-red-500 border-dashed p-2 mb-1 text-xs font-bold text-red-500">` +
+                `WARNING: The recipient address has ZERO transaction history. ` +
+                `This may indicate a fresh or unused address. Double-check the address before sending.</div>`;
+            if (warningsEl.classList.contains("hidden")) {
+                warningsEl.innerHTML = warning;
+                warningsEl.classList.remove("hidden");
+            } else {
+                warningsEl.innerHTML += warning;
+            }
+        }
+    } catch (e) {
+        log.errorf("recipient history check failed:", e.message);
+    }
+}
+
 function init(ctx) {
     $("btn-confirm-send").addEventListener("click", async () => {
         const password = $("confirm-tx-password").value;