fix: 补上缺失的 stopBtn DOM 元素，修复脚本崩溃

public/index.html

@@ -80,6 +80,7 @@
               </label>
               <div class="composer-hint">支持图片、文档、压缩包等</div>
               <div class="composer-actions single-line-actions">
+                <button type="button" id="stopBtn" class="stop-btn compact-send-btn hidden">停止</button>
                 <button type="submit" id="sendBtn" class="send-btn compact-send-btn">发送</button>
               </div>
             </div>