fix(client): flushNext в onFinish гейтится mountedRef (#486)

Финальный onFinish->flushNext() не проверял live-mount флаг. Чистый onFinish
может прийти ПОСЛЕ анмаунта треда (New-chat / переключение чата мид-стрим —
асинхронные attach/resume оседают поздно): flush дергал очередь и re-POST'ил
сообщение из брошенного треда — «призрачные» отправки/чаты-призраки. Остальные
обращения к очереди уже гейтятся mountedRef; закрываем последнюю дыру.

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

apps/client/src/features/ai-chat/components/chat-thread.test.tsx

@@ -203,6 +203,52 @@ describe("ChatThread — send now (#198)", () => {
   });
 });
 
+// #486: the final onFinish -> flushNext() must be gated on the live-mount flag.
+// A clean onFinish can land AFTER the thread unmounts (New-chat / chat-switch
+// mid-stream — the async attach/resume settles late); flushing then dequeues and
+// re-POSTs a queued message from an abandoned thread (a "ghost" send).
+describe("ChatThread — onFinish flush gated on mount (#486)", () => {
+  beforeEach(resetState);
+  afterEach(cleanup);
+
+  it("a clean onFinish WHILE MOUNTED flushes the queued message (control)", () => {
+    renderThread();
+    fireEvent.click(screen.getByTestId("queue-btn")); // enqueue "queued text"
+    expect(h.state.sendMessage).not.toHaveBeenCalled();
+
+    act(() => {
+      h.state.onFinish?.({
+        message: { id: "a", role: "assistant", parts: [] },
+        isAbort: false,
+        isDisconnect: false,
+        isError: false,
+      });
+    });
+    // Mounted: the queue flushes normally.
+    expect(h.state.sendMessage).toHaveBeenCalledWith({ text: "queued text" });
+  });
+
+  it("a clean onFinish AFTER unmount does NOT flush (no ghost send)", () => {
+    const { unmount } = renderThread();
+    fireEvent.click(screen.getByTestId("queue-btn")); // enqueue "queued text"
+    h.state.sendMessage.mockClear();
+
+    // Chat switched away mid-stream: the streamer unmounts...
+    unmount();
+    // ...and a late, clean onFinish lands on the abandoned thread.
+    act(() => {
+      h.state.onFinish?.({
+        message: { id: "a", role: "assistant", parts: [] },
+        isAbort: false,
+        isDisconnect: false,
+        isError: false,
+      });
+    });
+    // Gated on mountedRef: NOTHING is sent from the dead thread.
+    expect(h.state.sendMessage).not.toHaveBeenCalled();
+  });
+});
+
 // #396: in autonomous mode a live sendNow must additionally request the
 // AUTHORITATIVE server stop of the detached run (a local abort is only a client
 // disconnect the server ignores) and arm a bounded 409 retry so the re-POST

apps/client/src/features/ai-chat/components/chat-thread.tsx

@@ -659,7 +659,13 @@ export default function ChatThread({
         return;
       }
       if (isAbort || isDisconnect || isError) return;
-      flushNext();
+      // Gate the final flush on the live-mount flag (#486): a clean onFinish can
+      // land AFTER this thread unmounted (a New-chat / chat-switch mid-stream —
+      // the async attach/resume settles late). Flushing then dequeues and POSTs a
+      // queued message from an abandoned thread — a "ghost" send / ghost chat.
+      // Every other queue side effect already guards on mountedRef; this last one
+      // was the gap.
+      if (mountedRef.current) flushNext();
     },
     // `onError` runs in addition to `onFinish` (which ai@6 also calls on error).
     // Log the raw failure here for devtools; the UI shows a friendly classified