Add announcement banner for site transition update

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

Author: mkutmon

_layouts/default.html

@@ -31,6 +31,16 @@ <h2 class="project-tagline">Biological pathway creation and curation software</h
       <a href="https://pathvisio.github.io/faq" class="btn">FAQ</a>
       <a href="https://pathvisio.github.io/cite" class="btn">How to cite</a>
       <a href="https://pathvisio.github.io/contact" class="btn">Contact</a>
+      <!-- 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>
 
     <main id="content" class="main-content" role="main">