feat: 删除提问时级联删除其回答，并移除回答上的删除按钮

- 删除按钮只保留在用户提问气泡上；AI 回答不再单独提供删除
- 后端 DELETE /messages/{id} 改为按整轮删除：从提问删到下一条提问之前，
  返回实际删除的 deleted_ids，保持 checkpoint 合法（无孤立工具结果/回答）
- 前端隐藏后端返回的整批 id，删除后即时消失
- 更新删除相关中英文案

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend-web/src/components/workspace/messages/delete-message-button.tsx

@@ -17,10 +17,11 @@ import { useDeleteMessage } from "@/core/threads/hooks";
 import { Tooltip } from "../tooltip";
 
 /**
- * Hover affordance that deletes a single message (a user question or an
- * assistant answer) after a confirmation step. The actual removal is persisted
- * server-side; `onDeleted` lets the parent hide the bubble immediately without
- * waiting for the stream cache to refresh.
+ * Hover affordance that deletes a conversation turn (a user question together
+ * with its answer) after a confirmation step. The actual removal is persisted
+ * server-side; `onDeleted` receives the ids the server removed so the parent
+ * can hide those bubbles immediately without waiting for the stream cache to
+ * refresh.
  */
 export function DeleteMessageButton({
   threadId,
@@ -30,7 +31,7 @@ export function DeleteMessageButton({
 }: {
   threadId: string;
   messageId: string;
-  onDeleted?: () => void;
+  onDeleted?: (deletedIds: string[]) => void;
   className?: string;
 }) {
   const { t } = useI18n();
@@ -39,8 +40,11 @@ export function DeleteMessageButton({
 
   const handleConfirm = async () => {
     try {
-      await deleteMessage.mutateAsync({ threadId, messageId });
-      onDeleted?.();
+      const deletedIds = await deleteMessage.mutateAsync({
+        threadId,
+        messageId,
+      });
+      onDeleted?.(deletedIds);
       setOpen(false);
     } catch (error) {
       toast.error(

frontend-web/src/components/workspace/messages/message-list-item.tsx

@@ -147,18 +147,13 @@ export function MessageListItem({
   displaySources?: RuntimeDisplaySource[];
   extraRehypePlugins?: ReturnType<typeof useRehypeSplitWordsIntoSpans>;
   onEditResubmit?: (text: string) => void;
-  onMessageDeleted?: (messageId: string) => void;
+  onMessageDeleted?: (deletedIds: string[]) => void;
 }) {
   const isHuman = message.type === 'human';
   const messageMarkdown =
     extractContentFromMessage(message) ??
     extractReasoningContentFromMessage(message) ??
     '';
-  // Deletion is offered only for messages that already exist server-side
-  // (real, non-optimistic ids). Optimistic placeholders (`opt-*`) have no
-  // checkpoint entry to remove.
-  const canDelete =
-    !!onMessageDeleted && !!message.id && !message.id.startsWith('opt-');
   return (
     <AIElementMessage
       className={cn(
@@ -200,13 +195,6 @@ export function MessageListItem({
                 fileName="message"
               />
             )}
-            {canDelete && (
-              <DeleteMessageButton
-                threadId={threadId}
-                messageId={message.id!}
-                onDeleted={() => onMessageDeleted?.(message.id!)}
-              />
-            )}
           </div>
         </MessageToolbar>
       )}
@@ -389,7 +377,7 @@ function MessageContent_({
   displaySources?: RuntimeDisplaySource[];
   extraRehypePlugins?: ReturnType<typeof useRehypeSplitWordsIntoSpans>;
   onEditResubmit?: (text: string) => void;
-  onMessageDeleted?: (messageId: string) => void;
+  onMessageDeleted?: (deletedIds: string[]) => void;
 }) {
   const baseRehypePlugins = useRehypeSplitWordsIntoSpans(isLoading);
 
@@ -521,6 +509,9 @@ function MessageContent_({
   }
 
   if (isHuman) {
+    // Only the user's own question carries a delete affordance, and only once
+    // it exists server-side (real, non-optimistic id). Deleting it cascades to
+    // its answer on the backend.
     const canDelete =
       !!onMessageDeleted && !!message.id && !message.id.startsWith('opt-');
     return (
@@ -536,7 +527,7 @@ function MessageContent_({
             <DeleteMessageButton
               threadId={threadId}
               messageId={message.id!}
-              onDeleted={() => onMessageDeleted?.(message.id!)}
+              onDeleted={(ids) => onMessageDeleted?.(ids)}
               className="size-6"
             />
           ) : null

frontend-web/src/components/workspace/messages/message-list.tsx

@@ -104,10 +104,11 @@ export function MessageList({
   useEffect(() => {
     setHiddenMessageIds(new Set());
   }, [threadId]);
-  const handleMessageDeleted = useCallback((messageId: string) => {
+  const handleMessageDeleted = useCallback((deletedIds: string[]) => {
+    if (deletedIds.length === 0) return;
     setHiddenMessageIds((prev) => {
       const next = new Set(prev);
-      next.add(messageId);
+      for (const id of deletedIds) next.add(id);
       return next;
     });
   }, []);