Add announcement banner for Classic WikiPathways transition

Added an announcement banner regarding the transition of the Classic WikiPathways site to read-only status and the upcoming retirement of the site.

Author: mkutmon

_includes/header.html

@@ -48,4 +48,14 @@
     </div>
     </nav>
   </div>
-</header>    
+  <!-- ANNOUNCEMENT BANNER -->
+<div style="background-color: #fff3cd; color: #333; text-align: center; padding: 10px 20px; font-size: small; border-bottom: 1px solid #f0ad4e;">
+  ⚠️ <strong>Transition update:</strong>
+  As announced, the Classic WikiPathways site is now <strong>read-only</strong> and the old web service has been turned off.
+  <strong>From 1 May 2026, the Classic site will be fully retired</strong> and we will complete our transition to the new GitHub-based system.
+  If you are still using the old web service, please <a href="https://github.com/wikipathways/wikipathways-help/discussions" target="_blank" style="color: #333; text-decoration: underline;">contact us immediately</a> so we can help you transition.
+  &nbsp;|&nbsp;
+  <strong>Curators:</strong> PathVisio upload is currently not working.
+  If you need to upload or update a pathway, please <a href="https://github.com/wikipathways/wikipathways-help/discussions" target="_blank" style="color: #333; text-decoration: underline;">reach out</a> and we will help you find a solution. This should be resolved in the next few weeks.
+</div>
+</header>