feat: add etherscan link and click-to-copy on block number in success-tx view

Block numbers are blockchain entities like addresses and tx hashes. They now
receive the same treatment: click-to-copy and an external link icon pointing
to etherscan.io/block/{number}.

Closes #99

src/popup/views/txStatus.js

@@ -59,6 +59,16 @@ function txHashHtml(hash) {
     );
 }
 
+function blockNumberHtml(blockNumber) {
+    const num = String(blockNumber);
+    const link = `https://etherscan.io/block/${num}`;
+    const extLink = `<a href="${link}" target="_blank" rel="noopener" class="inline-flex items-center">${EXT_ICON}</a>`;
+    return (
+        `<span class="underline decoration-dashed cursor-pointer" data-copy="${escapeHtml(num)}">${escapeHtml(num)}</span>` +
+        extLink
+    );
+}
+
 function attachCopyHandlers(viewId) {
     document
         .getElementById(viewId)
@@ -189,7 +199,7 @@ function renderSuccess() {
         $("success-tx-to").innerHTML = toAddressHtml(d.to);
     }
 
-    $("success-tx-block").textContent = String(d.blockNumber);
+    $("success-tx-block").innerHTML = blockNumberHtml(d.blockNumber);
     $("success-tx-hash").innerHTML = txHashHtml(d.hash);
 
     // Show decoded calldata details if present