Optimize WebUI streaming and long history rendering

Batch stream deltas, window long transcripts, lazy-load syntax highlighting, and refine activity/composer interactions.

Add title refresh retries plus tests for streaming, windowing, code blocks, and live activity behavior.

webui/src/tests/useDeferredTitleRefresh.test.tsx

@@ -0,0 +1,110 @@
+import { act, renderHook } from "@testing-library/react";
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+
+import { useDeferredTitleRefresh } from "@/hooks/useDeferredTitleRefresh";
+import type { ChatSummary } from "@/lib/types";
+
+function session(overrides: Partial<ChatSummary> = {}): ChatSummary {
+  return {
+    key: "websocket:chat-a",
+    channel: "websocket",
+    chatId: "chat-a",
+    createdAt: null,
+    updatedAt: null,
+    title: "",
+    preview: "First user message",
+    ...overrides,
+  };
+}
+
+describe("useDeferredTitleRefresh", () => {
+  beforeEach(() => {
+    vi.useFakeTimers();
+  });
+
+  afterEach(() => {
+    vi.useRealTimers();
+  });
+
+  it("retries refreshing untitled sessions after turn_end", () => {
+    const refresh = vi.fn().mockResolvedValue(undefined);
+    const { result } = renderHook(() =>
+      useDeferredTitleRefresh(session(), refresh, [100, 300]),
+    );
+
+    act(() => {
+      result.current();
+    });
+
+    expect(refresh).toHaveBeenCalledTimes(1);
+
+    act(() => {
+      vi.advanceTimersByTime(100);
+    });
+    expect(refresh).toHaveBeenCalledTimes(2);
+
+    act(() => {
+      vi.advanceTimersByTime(200);
+    });
+    expect(refresh).toHaveBeenCalledTimes(3);
+  });
+
+  it("stops pending retries once a generated title arrives", () => {
+    const refresh = vi.fn().mockResolvedValue(undefined);
+    const { result, rerender } = renderHook(
+      ({ activeSession }) =>
+        useDeferredTitleRefresh(activeSession, refresh, [100, 300]),
+      { initialProps: { activeSession: session() } },
+    );
+
+    act(() => {
+      result.current();
+    });
+    rerender({ activeSession: session({ title: "Generated title" }) });
+
+    act(() => {
+      vi.advanceTimersByTime(300);
+    });
+
+    expect(refresh).toHaveBeenCalledTimes(1);
+  });
+
+  it("does not retry when the active session already has a title", () => {
+    const refresh = vi.fn().mockResolvedValue(undefined);
+    const { result } = renderHook(() =>
+      useDeferredTitleRefresh(session({ title: "Existing title" }), refresh, [100]),
+    );
+
+    act(() => {
+      result.current();
+      vi.advanceTimersByTime(100);
+    });
+
+    expect(refresh).toHaveBeenCalledTimes(1);
+  });
+
+  it("clears pending retries when the active chat changes", () => {
+    const refresh = vi.fn().mockResolvedValue(undefined);
+    const { result, rerender } = renderHook(
+      ({ activeSession }) =>
+        useDeferredTitleRefresh(activeSession, refresh, [100]),
+      { initialProps: { activeSession: session() } },
+    );
+
+    act(() => {
+      result.current();
+    });
+    rerender({
+      activeSession: session({
+        key: "websocket:chat-b",
+        chatId: "chat-b",
+      }),
+    });
+
+    act(() => {
+      vi.advanceTimersByTime(100);
+    });
+
+    expect(refresh).toHaveBeenCalledTimes(1);
+  });
+});