feat: add copy-flash visual feedback on click-to-copy

When a user clicks to copy text (addresses, tx hashes, etc.), the copied
element now briefly flashes with inverted colors (bg/fg swap) and fades
back over ~300ms. This provides localized visual feedback in addition to
the existing flash message.

Applied to all click-to-copy elements across all views.

closes #100

src/popup/views/transactionDetail.js

@@ -5,6 +5,7 @@ const {
     $,
     showView,
     showFlash,
+    flashCopyFeedback,
     addressDotHtml,
     addressTitle,
     escapeHtml,
@@ -171,6 +172,7 @@ function render() {
             el.onclick = () => {
                 navigator.clipboard.writeText(el.dataset.copy);
                 showFlash("Copied!");
+                flashCopyFeedback(el);
             };
         });
 }
@@ -248,6 +250,7 @@ async function loadCalldata(txHash, toAddress) {
                 el.onclick = () => {
                     navigator.clipboard.writeText(el.dataset.copy);
                     showFlash("Copied!");
+                    flashCopyFeedback(el);
                 };
             });
         }