Add API key warning to multi-user welcome dialog

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

frontend/index.html

@@ -553,7 +553,7 @@
             <input type="search" id="usernameInput" name="display_nickname" placeholder="Your name" maxlength="30" autocomplete="off" autocorrect="off" autocapitalize="off" spellcheck="false" data-1p-ignore data-lpignore="true" data-bwignore data-form-type="other" role="presentation">
             <div id="usernameWarning" class="username-warning" style="display:none"></div>
             <button id="usernameSubmit">Start</button>
-            <p class="username-notice">All sessions are publicly stored. For private use, <a href="https://github.com/huggingface/agent-ui" target="_blank">clone the repo</a> and run locally.</p>
+            <p class="username-notice"><strong>Important:</strong> All sessions are publicly stored. Do not add API keys to this space. For private use, <a href="https://github.com/huggingface/agent-ui" target="_blank">clone the repo</a> and run locally.</p>
         </div>
     </div>