docs: add Mistral AI footer attribution

Author: kiku-jw

docs/index.html

@@ -1039,7 +1039,8 @@ <h4>✅ Extracted Data</h4>
         <a href="https://github.com/kiku-jw/Chart2CSV/wiki">Documentation</a>
         <a href="https://github.com/kiku-jw/Chart2CSV/issues">Report Issue</a>
       </div>
-      <p>Built by <a href="https://github.com/kiku-jw">kiku-jw</a> • MIT License</p>
+      <p>Powered by <a href="https://mistral.ai">Mistral AI</a> • Built by <a
+          href="https://github.com/kiku-jw">kiku-jw</a> • MIT License</p>
     </div>
   </footer>