feat(web): per-topic 输入草稿记忆（切换暂存/恢复 + localStorage 持久化 + 删除清理）

- 文本草稿 debounce 持久化到 localStorage（key 按 topicId），附件仅存内存
- 切换话题时暂存当前草稿并恢复目标话题草稿，组件卸载时补齐落盘
- 删除话题时同步清除其持久化草稿，避免孤儿 key

web/src/components/Chat/MessageInput.tsx

@@ -15,6 +15,40 @@ import type { Attachment } from '../../types/protocol';
 
 const MAX_FILE_SIZE = 50 * 1024 * 1024; // 50MB
 
+/** per-topic 文本草稿的 localStorage key（附件 File 对象不可序列化，仅存内存） */
+export function draftStorageKey(topicId: string): string {
+  return `picobot:draft:${topicId}`;
+}
+
+function loadDraftText(topicId: string): string {
+  try {
+    return localStorage.getItem(draftStorageKey(topicId)) ?? '';
+  } catch {
+    return '';
+  }
+}
+
+function saveDraftText(topicId: string, text: string): void {
+  try {
+    if (text.trim()) {
+      localStorage.setItem(draftStorageKey(topicId), text);
+    } else {
+      localStorage.removeItem(draftStorageKey(topicId));
+    }
+  } catch {
+    // 隐私模式/配额满：静默降级为仅内存草稿
+  }
+}
+
+/** 删除话题时调用：清除其持久化草稿 */
+export function clearTopicDraft(topicId: string): void {
+  try {
+    localStorage.removeItem(draftStorageKey(topicId));
+  } catch {
+    // ignore
+  }
+}
+
 interface MessageInputProps {
   onSend: (content: string, attachments: Attachment[]) => void;
   onStop?: () => void;
@@ -24,7 +58,7 @@ interface MessageInputProps {
   isReadOnly?: boolean;
   channelName?: string;
   selectedExpert?: { name: string; description: string } | null;
-  /** 当前话题 ID，切换话题时自动清空草稿 */
+  /** 当前话题 ID：切换话题时保留各自草稿（文本持久化到 localStorage，附件仅内存） */
   topicId?: string | null;
 }
 
@@ -65,16 +99,77 @@ export function MessageInput({
   const fileInputRef = useRef<HTMLInputElement>(null);
   const wasLoadingRef = useRef(false);
   const prevTopicIdRef = useRef<string | null | undefined>(topicId);
+  // per-topic 草稿缓存：切换话题时暂存/恢复（附件 File 对象仅内存，刷新丢失）
+  const draftsRef = useRef<Map<string, { content: string; attachments: FileAttachment[] }>>(
+    new Map(),
+  );
+  const draftDebounceRef = useRef<number | null>(null);
 
-  // 切换话题时清空草稿（替代原来通过 key remount 的重置机制）
+  // 切换话题时：暂存当前草稿 → 恢复目标话题草稿（内存 miss 时读 localStorage）
   useEffect(() => {
-    if (prevTopicIdRef.current !== topicId) {
+    if (prevTopicIdRef.current === topicId) return;
+    const prevTopicId = prevTopicIdRef.current;
     prevTopicIdRef.current = topicId;
+
+    // 离开话题：立即持久化（补齐 debounce 未落盘部分）+ 暂存到内存（含附件）
+    if (prevTopicId != null) {
+      saveDraftText(prevTopicId, content);
+      draftsRef.current.set(prevTopicId, { content, attachments });
+    }
+
+    // 恢复目标话题草稿
+    if (topicId == null) {
       setContent('');
       setAttachments([]);
+      return;
     }
+    const cached = draftsRef.current.get(topicId);
+    if (cached) {
+      setContent(cached.content);
+      setAttachments(cached.attachments);
+    } else {
+      setContent(loadDraftText(topicId));
+      setAttachments([]);
+    }
+    // eslint-disable-next-line react-hooks/exhaustive-deps
   }, [topicId]);
 
+  // 文本草稿 debounce 持久化（300ms）
+  useEffect(() => {
+    if (topicId == null) return;
+    if (draftDebounceRef.current != null) {
+      window.clearTimeout(draftDebounceRef.current);
+    }
+    draftDebounceRef.current = window.setTimeout(() => {
+      saveDraftText(topicId, content);
+    }, 300);
+    return () => {
+      if (draftDebounceRef.current != null) {
+        window.clearTimeout(draftDebounceRef.current);
+      }
+    };
+  }, [content, topicId]);
+
+  // 卸载时补齐持久化（debounce 可能未落盘）+ 暂存内存草稿（含附件，供同 SPA 会话恢复）
+  const latestContentRef = useRef(content);
+  const latestAttachmentsRef = useRef(attachments);
+  latestContentRef.current = content;
+  latestAttachmentsRef.current = attachments;
+  useEffect(() => {
+    const drafts = draftsRef.current;
+    return () => {
+      const tid = prevTopicIdRef.current;
+      if (tid != null) {
+        saveDraftText(tid, latestContentRef.current);
+        drafts.set(tid, {
+          content: latestContentRef.current,
+          attachments: latestAttachmentsRef.current,
+        });
+      }
+    };
+    // eslint-disable-next-line react-hooks/exhaustive-deps
+  }, []);
+
   useEffect(() => {
     const textarea = textareaRef.current;
     if (textarea) {
@@ -250,6 +345,11 @@ export function MessageInput({
         content.trim(),
         attachments.map((a) => a.attachment),
       );
+      // 发送成功：清除该话题的草稿（内存 + localStorage）
+      if (topicId != null) {
+        draftsRef.current.delete(topicId);
+        saveDraftText(topicId, '');
+      }
       setContent('');
       setAttachments([]);
       setError(null);

web/src/hooks/chat/useTopics.ts

@@ -8,6 +8,7 @@ import {
   type MutableRefObject,
 } from 'react';
 import type { Topic, TopicList, TopicRenamed, TopicSummary, Command } from '../../types/protocol';
+import { clearTopicDraft } from '../../components/Chat/MessageInput';
 
 export interface UseTopicsReturn {
   topics: Topic[];
@@ -101,6 +102,8 @@ export function useTopics(): UseTopicsReturn {
   }, []);
 
   const deleteTopic = useCallback((topicId: string): Command => {
+    // 同步清除该话题的持久化草稿（删除后端话题成功与否均无碍：残留草稿 key 无话题可挂载）
+    clearTopicDraft(topicId);
     return { type: 'delete_topic', topic_id: topicId };
   }, []);