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>

src/widgets/chat-message.jsx

@@ -65,6 +65,7 @@ class BaseChatMessage extends React.PureComponent {
     this.handleTouchStart = this.handleTouchStart.bind(this);
     this.handleTouchMove = this.handleTouchMove.bind(this);
     this.handleTouchEnd = this.handleTouchEnd.bind(this);
+    this.suppressNativeContextMenu = this.suppressNativeContextMenu.bind(this);
     this.handleCancelUpload = this.handleCancelUpload.bind(this);
     this.handleDraftyClick = this.handleDraftyClick.bind(this);
     this.handleVoicePlayed = this.handleVoicePlayed.bind(this);
@@ -208,6 +209,14 @@ class BaseChatMessage extends React.PureComponent {
     this.longPressOrigin = null;
   }
 
+  // The press-and-hold gesture above is our own timer-based approximation of "long press" -- it
+  // doesn't stop the browser's own native context menu (right-click on desktop, or the OS-level
+  // long-press menu some browsers still fire as a real `contextmenu` event even for a touch/mouse
+  // hold). Left alone, both menus show up stacked on top of each other.
+  suppressNativeContextMenu(e) {
+    e.preventDefault();
+  }
+
   componentWillUnmount() {
     this.cancelLongPress();
   }
@@ -472,7 +481,8 @@ class BaseChatMessage extends React.PureComponent {
           <i className="material-icons red message-fail-icon">error</i> : null}
         <div>
           <div className={bubbleClass}
-            onTouchStart={this.handleTouchStart} onTouchMove={this.handleTouchMove} onTouchEnd={this.handleTouchEnd}>
+            onTouchStart={this.handleTouchStart} onTouchMove={this.handleTouchMove} onTouchEnd={this.handleTouchEnd}
+            onContextMenu={this.suppressNativeContextMenu}>
             {showVoiceUnreadDot ? <i className="voice-unread-dot" /> : null}
             <div className="content-meta">
               <div className={textSizeClass}>