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/confirmTx.js

@@ -15,6 +15,7 @@ const {
     hideError,
     showView,
     showFlash,
+    flashCopyFeedback,
     addressTitle,
     addressDotHtml,
     escapeHtml,
@@ -117,6 +118,7 @@ function show(txInfo) {
             copyEl.onclick = () => {
                 navigator.clipboard.writeText(copyEl.dataset.copy);
                 showFlash("Copied!");
+                flashCopyFeedback(copyEl);
             };
         }
     } else {