排查转发bug同类问题：修复查找聊天记录崩溃 + 复制功能复制出"undefined"

审查了所有"读取消息content做数据处理"的地方，找到两处和转发bug同一根因
(手机端扁平非Drafty内容格式)的真实问题：

1. 查找聊天记录(HistorySearchModal)的搜索结果渲染，对手机端发送的图片/文件/
   语音/视频/名片消息命中关键词时会让整个应用崩溃——Drafty.toPlainText在这
   类扁平内容上返回undefined，highlightMatches对undefined调用.toLowerCase()
   直接抛异常，而这个应用完全没有错误边界(ErrorBoundary)兜底，等于白屏。
   已用真实搜索接口返回确认snapshot里其实带着head字段(此前SearchHit类型
   没建模)，据此在渲染前识别这几种扁平内容并显示[图片]/[文件]/[语音]/[名片]
   这样的占位文案，不再无脑喂给renderDrafty；顺手给highlightMatches补了防御
   性判空，避免类似疏漏再次崩溃整个应用。

2. 消息右键菜单"复制"功能，对手机端发送的扁平文本消息(如群公告自动发布的
   那条)和图片/文件/语音/视频消息，复制到剪贴板的是字面字符串"undefined"。
   文本类修成正确提取真实文本；附件类直接从"复制"这个操作里去掉(参考名片/
   通话记录消息已有的排除逻辑)，复制一段JSON式的附件数据本来就没有意义。

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

web/src/features/messages/ConversationView.tsx

@@ -24,7 +24,7 @@ import { getMe } from '@/lib/api/profile';
 import { FETCH_PAGE_TRIGGER, MESSAGES_PAGE } from '@/lib/config';
 import { useClickOutside } from '@/lib/hooks';
 import { availableActions, recallRemainingSeconds, type MessageAction } from '@/lib/messages/actions';
-import { isRecalled } from '@/lib/messages/dispatch';
+import { flatTextContentOf, isRecalled } from '@/lib/messages/dispatch';
 import { buildMentionedContent, detectMentionTrigger, type PendingMention } from '@/lib/messages/mentions';
 import { buildReplyInfo, replySnapshotText } from '@/lib/messages/reply';
 import { groupSenderLabel } from '@/lib/messages/sender-name';
@@ -846,9 +846,16 @@ export function ConversationView() {
     setMenu(null);
     if (!msg || typeof msg.seq !== 'number') return;
     switch (action) {
-      case 'copy':
-        navigator.clipboard.writeText(Drafty.toPlainText(msg.content ?? '')).catch(() => {});
+      case 'copy': {
+        // Real, live-confirmed bug: a flat non-Drafty text message (e.g. the group-announcement
+        // auto-post, `{text:"..."}` not `{txt:"..."}`) isn't excluded from 'copy' by availableActions
+        // (it reads fine in the bubble via renderDrafty's own flatTextContentOf fallback -- only this
+        // handler's `Drafty.toPlainText` didn't know about it), so "复制" silently put the literal
+        // string "undefined" on the clipboard instead of the text visibly shown on screen.
+        const flatText = flatTextContentOf(msg.content);
+        navigator.clipboard.writeText(flatText ?? Drafty.toPlainText(msg.content ?? '')).catch(() => {});
         break;
+      }
       case 'recall':
         recallMessage(topicId, msg.seq)
           .then(() => topic.delMessagesList([msg.seq], true))