feat: show red warning on confirm-tx for addresses with zero transaction history

When sending to an address that has never sent or received funds (zero
nonce and zero balance), display a prominent red warning on the
transaction confirmation screen advising the user to double-check the
address.

Closes #82

src/popup/views/confirmTx.js

@@ -86,6 +86,42 @@ function valueWithUsd(text, usdAmount) {
     return text;
 }
 
+function renderWarnings(warnings) {
+    const warningsEl = $("confirm-warnings");
+    if (warnings.length > 0) {
+        warningsEl.innerHTML = warnings
+            .map(
+                (w) =>
+                    `<div class="border border-border border-dashed p-2 mb-1 text-xs font-bold" style="color:#c00">WARNING: ${w}</div>`,
+            )
+            .join("");
+        warningsEl.classList.remove("hidden");
+    } else {
+        warningsEl.classList.add("hidden");
+    }
+}
+
+async function checkAddressHistory(address, existingWarnings) {
+    try {
+        const provider = getProvider(state.rpcUrl);
+        const [balance, txCount] = await Promise.all([
+            provider.getBalance(address),
+            provider.getTransactionCount(address),
+        ]);
+        if (balance === 0n && txCount === 0) {
+            const warnings = existingWarnings.slice();
+            warnings.push(
+                "This address has ZERO transaction history. " +
+                    "It has never sent or received funds. " +
+                    "Double-check that the address is correct before sending.",
+            );
+            renderWarnings(warnings);
+        }
+    } catch (e) {
+        log.errorf("address history check failed:", e.message);
+    }
+}
+
 function show(txInfo) {
     pendingTx = txInfo;
 
@@ -176,18 +212,10 @@ function show(txInfo) {
         warnings.push("You are sending to your own address.");
     }
 
-    const warningsEl = $("confirm-warnings");
-    if (warnings.length > 0) {
-        warningsEl.innerHTML = warnings
-            .map(
-                (w) =>
-                    `<div class="border border-border border-dashed p-2 mb-1 text-xs font-bold">WARNING: ${w}</div>`,
-            )
-            .join("");
-        warningsEl.classList.remove("hidden");
-    } else {
-        warningsEl.classList.add("hidden");
-    }
+    renderWarnings(warnings);
+
+    // Async check: warn if destination address has zero transaction history
+    checkAddressHistory(txInfo.to, warnings);
 
     // Check for errors
     const errors = [];