feat: 支持删除单条聊天消息（用户提问与模型回答）

- 后端新增 DELETE /api/threads/{thread_id}/messages/{message_id}，从最新 checkpoint 的 messages 通道过滤掉指定 id 并写回新 checkpoint
- 前端 useDeleteMessage 钩子 + DeleteMessageButton（带确认框）组件
- 在用户问题气泡与助手回答工具栏上各加删除按钮，仅对已落库消息显示
- MessageList 本地隐藏已删消息并在切换会话时重置；只读/分享场景禁用删除
- 新增 deleteMessage 相关中英文案

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

frontend-web/src/core/threads/hooks.ts

@@ -350,12 +350,12 @@ export function useThreadStream({
         // agent thread URL — otherwise fall back to the global new chat.
         const hash = typeof window !== "undefined" ? window.location.hash : "";
         const agentMatch = hash.match(/#\/page\/workspace\/agents\/([^/]+)\//);
-        const iframeMatch = hash.match(/#\/page\/workspace\/chats_iframe\//);
-        const target = agentMatch
-          ? `/page/workspace/agents/${agentMatch[1]}/chats/new`
-          : iframeMatch
-            ? "/page/workspace/chats_iframe/new"
-            : "/page/workspace/chats/new";
+        const iframeMatch = hash.match(/#\/page\/workspace\/chats_iframe\//);
+        const target = agentMatch
+          ? `/page/workspace/agents/${agentMatch[1]}/chats/new`
+          : iframeMatch
+            ? "/page/workspace/chats_iframe/new"
+            : "/page/workspace/chats/new";
         // First navigate to a safe URL so the post-reload page lands on a
         // valid route. Then full-reload so TanStack Query cache, context
         // providers and any other state captured under the previous user's
@@ -804,6 +804,41 @@ export function useDeleteThread() {
   });
 }
 
+/**
+ * Delete a single message (a user question or an assistant answer) from a
+ * thread. Hits the Gateway endpoint that filters the message out of the latest
+ * checkpoint. The live `useStream` cache is not auto-refreshed by this call —
+ * callers hide the deleted id locally (see `MessageList`) so the removal shows
+ * immediately; a reload then reads the persisted, already-trimmed state.
+ */
+export function useDeleteMessage() {
+  return useMutation({
+    mutationFn: async ({
+      threadId,
+      messageId,
+    }: {
+      threadId: string;
+      messageId: string;
+    }) => {
+      const response = await apiFetch(
+        `${getBackendBaseURL()}/api/threads/${encodeURIComponent(
+          threadId,
+        )}/messages/${encodeURIComponent(messageId)}`,
+        {
+          method: "DELETE",
+        },
+      );
+
+      if (!response.ok) {
+        const error = await response
+          .json()
+          .catch(() => ({ detail: "删除消息失败。" }));
+        throw new Error(error.detail ?? "删除消息失败。");
+      }
+    },
+  });
+}
+
 export function useRenameThread() {
   const queryClient = useQueryClient();
   const apiClient = getAPIClient();