Add footer with CHNS.tech credit and Buy Me a Coffee button

Adds a green footer (matching nav colour) to all authenticated pages
with a "Created by: CHNS.tech" link and a styled BMC button. CSP updated
to allow buymeacoffee CDN domains.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

nginx/html/admin.html

@@ -77,6 +77,11 @@
         </div>
     </main>
 
+    <footer class="site-footer">
+        Created by: <a href="https://chns.tech" target="_blank" rel="noopener noreferrer">CHNS.tech</a>
+        <a href="https://www.buymeacoffee.com/CHNS" target="_blank" rel="noopener noreferrer" class="bmc-btn">🎉 Buy me an energy drink</a>
+    </footer>
+
     <script src="/js/api.js?v=4"></script>
     <script src="/js/auth.js?v=4"></script>
     <script src="/js/admin.js?v=4"></script>