修复长按消息气泡弹出自定义菜单时，浏览器/系统原生右键菜单跟着一起弹出的问题

chat-message.jsx 新增 suppressNativeContextMenu，在气泡上监听 contextmenu 事件并 preventDefault，
避免长按（或桌面端右键/触屏模拟长按）触发的原生上下文菜单和应用自己的长按菜单重叠显示。
css/base.css 给 .bubble 补了 -webkit-touch-callout:none，同时覆盖 iOS Safari 那条独立于
contextmenu 事件之外的长按呼出菜单（复制/查询/分享...），不影响 user-select，桌面端正常的
点击拖拽选字复制不受影响。

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

css/base.css

@@ -4586,6 +4586,11 @@ video {
   transition: ease-out background-color 0.5s;
   transition-delay: 0.3s;
   filter: drop-shadow(0.5px 0.5px 0.7px var(--clr-shadow-bubble-drop));
+  /* iOS Safari's press-and-hold callout (Copy/Look Up/Share...) is a separate mechanism from the
+     `contextmenu` DOM event suppressed in chat-message.jsx -- needs its own opt-out so it doesn't
+     stack with our own long-press menu. Doesn't touch user-select, so normal click-drag text
+     selection on desktop is untouched. */
+  -webkit-touch-callout: none;
 }
 
 .chat-box li.single {