fix: make heading anchors copy full URL to clipboard fixes #6756 (#7099)

### Summary
This PR fixes issue #6756 where clicking the '#' anchor next to headings
did not copy the section link to the clipboard.  

### Changes
- Updated `HeaderWrapperExtension` to add an inline `onclick` handler
  for the heading anchor `<a>` elements.
- Clicking '#' now:
  - Copies the full URL including fragment to the clipboard
  - Updates the URL in the address bar
  - Prevents default page jump behavior
- Temporarily replaces '#' with a smaller 'Copied!' text for 1.2 seconds
to provide visual feedback

### Testing
- Verified that clicking '#' on h2–h5 headings copies the correct URL
- Confirmed that the page URL updates without scrolling
- Works across multiple headings on the same page

### Notes
- This approach adds behavior directly in the anchor element.
- Alternative approach would be a global client-side handler, but this
  is safe and consistent with current Jaspr HTML generation.

Fixes #6756 

reference video: 


https://github.com/user-attachments/assets/06411e8d-07c2-4057-a20e-ace125897a16

---------

Co-authored-by: Parker Lougheed <parlough@gmail.com>
Co-authored-by: Connie Ooi <connieooi@google.com>

Author: 3 people

site/lib/src/client/global_scripts.dart

@@ -5,6 +5,7 @@
 import 'dart:convert';
 
 import 'package:http/http.dart' as http;
+import 'package:meta/meta.dart';
 import 'package:universal_web/js_interop.dart';
 import 'package:universal_web/web.dart' as web;
 
@@ -13,6 +14,7 @@ import 'package:universal_web/web.dart' as web;
 /// These are temporary until they can be integrated with their
 /// relevant Jaspr components.
 void setUpSite() {
+  _setUpHeadingLinkCopy();
   _setUpSidenav();
   _setUpSearchKeybindings();
   _setUpTabs();
@@ -24,6 +26,56 @@ void setUpSite() {
   _setUpSteppers();
 }
 
+void _setUpHeadingLinkCopy() {
+  final anchors = web.document.querySelectorAll(
+    'a.heading-link[href]',
+  );
+
+  for (var i = 0; i < anchors.length; i++) {
+    final anchor = anchors.item(i) as web.HTMLAnchorElement;
+
+    // Store the original text before any click handlers modify it.
+    final originalText = anchor.textContent;
+
+    @awaitNotRequired
+    Future<void> copyAnchorToClipboard(String url) async {
+      try {
+        await web.window.navigator.clipboard.writeText(url).toDart;
+        // TODO: Replace this with a tooltip of some sort.
+        anchor.textContent = 'Link copied!';
+
+        Future<void>.delayed(const Duration(milliseconds: 1200), () {
+          anchor.textContent = originalText;
+        });
+      } catch (_) {
+        anchor.textContent = 'Failed to copy';
+      }
+    }
+
+    anchor.addEventListener(
+      'click',
+      ((web.Event event) {
+        event.preventDefault();
+
+        final fragment = anchor.hash;
+        if (!fragment.startsWith('#')) return;
+
+        final headingId = fragment.substring(1);
+        if (headingId.isEmpty) return;
+
+        final url = Uri.parse(
+          web.window.location.href,
+        ).replace(fragment: headingId).toString();
+
+        // Update the URL hash without triggering a scroll jump.
+        web.window.history.replaceState(null, '', '#$headingId');
+
+        copyAnchorToClipboard(url);
+      }).toJS,
+    );
+  }
+}
+
 void _setUpSidenav() {
   final sidenav = web.document.getElementById('sidenav');
   if (sidenav == null) return;