fix(frontend): fix new-chat navigation stale state issue (#1077)

- Use router.replace() instead of history.replaceState() so Next.js
  router's internal state is updated on chat start. This ensures
  subsequent "New Chat" clicks are treated as a real cross-route
  navigation (actual-id → "new") rather than a no-op same-path
  navigation, which was causing stale content to persist.

- In ChatLayout, increment the SubtasksProvider key only when
  navigating TO "new" from a non-"new" route. This forces a full
  remount for a fresh new-chat state without remounting when the URL
  transitions from "new" → actual-id (which would interrupt streaming).

Made-with: Cursor

Co-authored-by: DanielWalnut <45447813+hetaoBackend@users.noreply.github.com>

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

@@ -1,5 +1,8 @@
 "use client";
 
+import { useParams } from "next/navigation";
+import { useEffect, useRef, useState } from "react";
+
 import { PromptInputProvider } from "@/components/ai-elements/prompt-input";
 import { ArtifactsProvider } from "@/components/workspace/artifacts";
 import { SubtasksProvider } from "@/core/tasks/context";
@@ -9,8 +12,24 @@ export default function ChatLayout({
 }: {
   children: React.ReactNode;
 }) {
+  const { thread_id } = useParams<{ thread_id: string }>();
+  const prevThreadId = useRef(thread_id);
+
+  // Increment only when navigating TO "new" from a non-"new" route.
+  // This forces a full remount of the subtree for a fresh new-chat state,
+  // without remounting when the URL transitions from "new" → actual-id
+  // (which would interrupt streaming).
+  const [generation, setGeneration] = useState(0);
+
+  useEffect(() => {
+    if (thread_id === "new" && prevThreadId.current !== "new") {
+      setGeneration((g) => g + 1);
+    }
+    prevThreadId.current = thread_id;
+  }, [thread_id]);
+
   return (
-    <SubtasksProvider>
+    <SubtasksProvider key={generation}>
       <ArtifactsProvider>
         <PromptInputProvider>{children}</PromptInputProvider>
       </ArtifactsProvider>