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/addressDetail.js

@@ -2,6 +2,7 @@ const {
     $,
     showView,
     showFlash,
+    flashCopyFeedback,
     balanceLinesForAddress,
     addressDotHtml,
     addressTitle,
@@ -241,6 +242,7 @@ function init(_ctx) {
         if (addr) {
             navigator.clipboard.writeText(addr);
             showFlash("Copied!");
+            flashCopyFeedback($("address-full"));
         }
     });
 
@@ -358,6 +360,7 @@ function init(_ctx) {
         if (key) {
             navigator.clipboard.writeText(key);
             showFlash("Copied!");
+            flashCopyFeedback($("export-privkey-value"));
         }
     });
 
@@ -366,6 +369,7 @@ function init(_ctx) {
         if (full) {
             navigator.clipboard.writeText(full);
             showFlash("Copied!");
+            flashCopyFeedback($("export-privkey-address"));
         }
     });