修复群公告发布通知消息正文完全不显示、聊天列表预览显示"undefined"

群公告发布后自动生成的通知消息，真实content是{text:"群公告\n<内容>"}这种
扁平对象（不是纯字符串，也不是标准Drafty的{txt,fmt,ent}结构），且
head['x-im-type']是"text"而不是已处理的"system_notice"，落到Drafty处理
分支后既渲染不出气泡正文、列表预览也变成字面的"undefined"字符串。

抽出dispatch.ts里原本只服务system_notice的{text}扁平内容识别逻辑
（重命名为flatTextContentOf并导出），在renderDrafty和
formatMessagePreview的Drafty分支之前都接上——不改变已有的居中系统提示
样式（这条消息本来就该是普通气泡，发送者信息一直是正确的，只是正文
丢了），只是把"文本内容是扁平{text}对象"这一种情况也当作合法纯文本处理。

实测：真实发布一条群公告后，气泡正文和列表预览都正确显示"群公告
全功能测试公告内容"，不再是空白或"undefined"。

web/src/lib/messages/dispatch.ts

@@ -18,12 +18,28 @@ export function isRecalled(msg: StoredMessage): boolean {
   return !isServerDeleted(msg) && msg.content == null;
 }
 
-interface SystemNoticeContent {
+interface FlatTextContent {
   text: string;
 }
 
-function isSystemNoticeContent(content: unknown): content is SystemNoticeContent {
-  return !!content && typeof content === 'object' && typeof (content as SystemNoticeContent).text === 'string';
+function isFlatTextContent(content: unknown): content is FlatTextContent {
+  return !!content && typeof content === 'object' && typeof (content as FlatTextContent).text === 'string';
+}
+
+/**
+ * `{text: "..."}` -- a flat, non-Drafty content shape (no `txt`/`fmt`/`ent`) this backend uses for
+ * more than one kind of message, not just the centered system-event lines `systemNoticeText` below
+ * renders: a real, live-confirmed example is the auto-posted "群公告\n<announcement text>" message
+ * a group's announcement publish creates, which has an ordinary sender/avatar (NOT a centered system
+ * line) and `head['x-im-type']:"text"` (not `system_notice`) -- but the SAME flat `{text}` content
+ * shape regardless. `renderDrafty`/`formatMessagePreview` both need this recognized as plain text
+ * before falling through to real Drafty processing (`Drafty.format`/`Drafty.preview` on an object
+ * with none of the fields they expect renders empty/`undefined`, confirmed live: a group's own
+ * announcement-notice bubble showed no text at all, and the chat list's preview literally read "我：
+ * undefined").
+ */
+export function flatTextContentOf(content: unknown): string | null {
+  return isFlatTextContent(content) ? content.text : null;
 }
 
 /**
@@ -35,7 +51,7 @@ function isSystemNoticeContent(content: unknown): content is SystemNoticeContent
 export function systemNoticeText(msg: StoredMessage): string | null {
   const headType = msg.head?.['x-im-type'];
   if (headType !== 'system_notice' && headType !== 'system_action') return null;
-  return isSystemNoticeContent(msg.content) ? msg.content.text : null;
+  return flatTextContentOf(msg.content);
 }
 
 export interface ContactCardContent {

web/src/lib/messages/drafty.tsx

@@ -4,6 +4,7 @@ import { Drafty, type DraftyContent } from 'tinode-sdk';
 
 import { AudioAttachment, FileAttachment, ImageAttachment, VideoAttachment } from '@/features/messages/AttachmentPreview';
 import { SafeLink } from '@/features/messages/SafeLink';
+import { flatTextContentOf } from '@/lib/messages/dispatch';
 
 /**
  * Bubble content renderer for the Drafty style tags this app currently sends/receives. Grown
@@ -124,6 +125,8 @@ function makeFormatNode(highlightQuery: string | undefined) {
 export function renderDrafty(content: DraftyContent | string | null | undefined, highlightQuery?: string): ReactNode {
   if (content == null) return null;
   if (typeof content === 'string') return highlightQuery ? <>{highlightMatches(content, highlightQuery, 'plain')}</> : content;
+  const flatText = flatTextContentOf(content);
+  if (flatText != null) return highlightQuery ? <>{highlightMatches(flatText, highlightQuery, 'plain')}</> : flatText;
   if (Drafty.isPlainText(content)) {
     const text = Drafty.toPlainText(content);
     return highlightQuery ? <>{highlightMatches(text, highlightQuery, 'plain')}</> : text;

web/src/lib/messages/preview.ts

@@ -1,6 +1,6 @@
 import { Drafty, type DraftyContent, type StoredMessage } from 'tinode-sdk';
 
-import { callResultDisplay, contactCardContentOf, isServerDeleted, systemNoticeText } from '@/lib/messages/dispatch';
+import { callResultDisplay, contactCardContentOf, flatTextContentOf, isServerDeleted, systemNoticeText } from '@/lib/messages/dispatch';
 
 /**
  * One-line chat-list preview text (README §2.1): plain text renders as-is, everything else falls
@@ -26,6 +26,13 @@ export function formatMessagePreview(msg: StoredMessage | undefined, myUserId: s
   if (contactCardContentOf(msg)) return `${prefix}[名片]`;
   const callResult = callResultDisplay(msg, msg.from === myUserId);
   if (callResult) return `${prefix}${callResult.text}`;
+  // Same flat `{text}` shape `systemNoticeText` above already knows about, but NOT gated to
+  // `system_notice`/`system_action` -- a real, live-confirmed example is a group's own
+  // announcement-publish notice (`head['x-im-type']:"text"`, ordinary sender, not a centered system
+  // line), which fell through everything above straight into the Drafty-entity branches below and
+  // produced the literal string "undefined" (confirmed live: chat list showed "我：undefined").
+  const flatText = flatTextContentOf(content);
+  if (flatText != null) return prefix + flatText;
   if (typeof content === 'string' || Drafty.isPlainText(content)) {
     return prefix + Drafty.toPlainText(content);
   }