修复撤回消息在对方界面显示"无效内容"而非"消息已删除"

现象：真实调用撤回接口成功后（recall_window_seconds=120秒窗口内，长按菜单
"撤回"），撤回发起方自己的消息列表里这条消息直接消失，没有问题；但对方
（消息接收方）的聊天界面上，这条消息位置渲染成一个灰色警告图标+"无效内容"
（invalid content），不是预期的"此消息已被撤回/删除"占位提示。

根因：Tinode原生协议里，一条消息被撤回后，服务端把这条消息的content原地
置为null（seq位置保留，其余内容清空）。chat-message.jsx渲染消息内容时按
Drafty富文本、纯字符串、其余情况三段分支处理，content为null时两段判断
（Drafty.isValid和typeof=='string'）都不成立，直接落到兜底的"invalid
content"分支——这段代码从未加过null-content（撤回）的专门判断。这跟
messages-view.jsx里另一套依赖msg.hi标记位的"删除"消息渲染（走独立的
<MetaMessage deletedText=.../>组件）是两码事：撤回消息还是走的普通
ChatMessage组件，从未接入deletedText这个机制。

修复：chat-message.jsx新增一个content==null的分支，复用
messages-view.jsx里messages.message_deleted已经注册好的同一个i18n
id/文案（"消息已删除"，非新增翻译串），渲染为灰色图标+"消息已删除"，
不再落到"无效内容"兜底分支。

验证：真实浏览器操作，两个全新注册账号互为好友，A发一条文本消息后
立即长按撤回（120秒窗口内），B的聊天界面上这条消息正确显示"消息已删除"
（此前是"无效内容"）。

第14+轮最小粒度测试 Ch4 发现（撤回消息渲染场景）。

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

src/widgets/chat-message.jsx

@@ -374,6 +374,23 @@ class BaseChatMessage extends React.PureComponent {
         // Content consists of 1-5 emoji characters. Count how many and use it to increase the font size.
         textSizeClass += ' emoji-' + (content || '').match(/\p{RGI_Emoji}/vg).length;
       }
+    } else if (content == null) {
+      // Tinode's wire protocol marks a recalled message by nulling its `content` in place (the seq
+      // slot stays, everything else is gone) -- see server's message.recalled handling and
+      // IM-前后端对接文档.md 1256行. That's a *different* deletion shape from the msg.hi-flagged kind
+      // messages-view.jsx already handles by rendering a dedicated <MetaMessage deleted deletedText=.../>
+      // instead of a ChatMessage at all (deletedText is that component's own prop, not this one's --
+      // there's no equivalent plumbed in here). Recalled messages still go through ordinary ChatMessage,
+      // so without this branch a null content fell all the way through to the generic "invalid content"
+      // case below -- confirmed live: recalling a message left the *other* party's chat showing a bare
+      // "⚠ invalid content" (无效内容) row where "消息已删除" belongs (the recalling side just drops the
+      // row from its own list locally, unaffected by this). Reuses the exact same i18n id/defaultMessage
+      // messages-view.jsx's `messages.message_deleted` already registers (translated zh string:
+      // "消息已删除"), not a new string. `content == null` (not `===`) intentionally also catches
+      // `undefined`, same as every other falsy-content branch here.
+      content = <><i className="material-icons gray">block</i> <i className="gray">
+        <FormattedMessage id="message_deleted" defaultMessage="message deleted"
+          description="Placeholder shown in the timeline where a message was recalled or deleted" /></i></>;
     } else {
       content = <><i className="material-icons gray">warning_amber</i> <i className="gray">
         <FormattedMessage id="invalid_content"