fix(frontend): hide history reliably in welcome mode

frontend/src/app/workspace/chats/[thread_id]/page.tsx

@@ -135,10 +135,16 @@ export default function ChatPage() {
       setHistoryCutoff(null);
       return;
     }
-    if (historyCutoff === null && !thread.isThreadLoading) {
-      setHistoryCutoff(thread.messages.length);
-    }
+    if (hasSubmitted) return;
+    // Welcome 态下、未提交前，把当前已有消息都当作“历史”切掉。
+    // 这样即使历史消息是后续异步补齐，也不会重新露出。
+    setHistoryCutoff((prev) => {
+      const next = thread.messages.length;
+      if (prev === null) return next;
+      return next > prev ? next : prev;
+    });
   }, [
+    hasSubmitted,
     historyCutoff,
     shouldRenderHistory,
     thread.isThreadLoading,
@@ -331,9 +337,11 @@ export default function ChatPage() {
                     threadId={threadId}
                     thread={thread}
                     messagesOverride={
-                      shouldRenderHistory || historyCutoff === null
+                      shouldRenderHistory
                         ? undefined
-                        : thread.messages.slice(historyCutoff)
+                        : historyCutoff === null
+                          ? []
+                          : thread.messages.slice(historyCutoff)
                     }
                     paddingBottom={todoListCollapsed ? 160 : 280}
                     showScrollToBottomButton={!showWelcomeStyle}