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/helpers.js

@@ -258,12 +258,26 @@ function timeAgo(timestamp) {
     return years + " year" + (years !== 1 ? "s" : "") + " ago";
 }
 
+function flashCopyFeedback(el) {
+    if (!el) return;
+    el.classList.remove("copy-flash-fade");
+    el.classList.add("copy-flash-active");
+    setTimeout(() => {
+        el.classList.remove("copy-flash-active");
+        el.classList.add("copy-flash-fade");
+        setTimeout(() => {
+            el.classList.remove("copy-flash-fade");
+        }, 350);
+    }, 100);
+}
+
 module.exports = {
     $,
     showError,
     hideError,
     showView,
     showFlash,
+    flashCopyFeedback,
     balanceLine,
     balanceLinesForAddress,
     addressColor,