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/code-block.test.tsx

@@ -0,0 +1,92 @@
+import { act, render, screen } from "@testing-library/react";
+import { describe, expect, it, vi } from "vitest";
+
+import { CodeBlock } from "@/components/CodeBlock";
+import { ThemeProvider } from "@/hooks/useTheme";
+
+const mockedStyles = vi.hoisted(() => ({
+  dark: { pre: { background: "#111" } },
+  light: { pre: { background: "#fff" } },
+}));
+
+vi.mock("react-syntax-highlighter/dist/esm/prism-async-light", () => ({
+  default: ({
+    children,
+    style,
+  }: {
+    children: string;
+    style: Record<string, unknown>;
+  }) => (
+    <pre
+      data-testid="highlighted-code"
+      data-theme={style === mockedStyles.dark ? "dark" : "light"}
+    >
+      <code>{children}</code>
+    </pre>
+  ),
+}));
+
+vi.mock("react-syntax-highlighter/dist/esm/styles/prism/one-dark", () => ({
+  default: mockedStyles.dark,
+}));
+
+vi.mock("react-syntax-highlighter/dist/esm/styles/prism/one-light", () => ({
+  default: mockedStyles.light,
+}));
+
+describe("CodeBlock", () => {
+  it("reads theme from context without creating per-block observers", async () => {
+    const originalMutationObserver = globalThis.MutationObserver;
+    const observer = vi.fn();
+    class MockMutationObserver {
+      constructor(callback: MutationCallback) {
+        observer(callback);
+      }
+
+      observe = vi.fn();
+
+      disconnect = vi.fn();
+
+      takeRecords() {
+        return [];
+      }
+    }
+    vi.stubGlobal("MutationObserver", MockMutationObserver);
+
+    try {
+      const { rerender } = render(
+        <ThemeProvider theme="dark">
+          <CodeBlock language="ts" code="const value = 1;" />
+        </ThemeProvider>,
+      );
+
+      await act(async () => {
+        await Promise.resolve();
+        await Promise.resolve();
+      });
+
+      expect(screen.getByTestId("highlighted-code")).toHaveAttribute(
+        "data-theme",
+        "dark",
+      );
+
+      rerender(
+        <ThemeProvider theme="light">
+          <CodeBlock language="ts" code="const value = 1;" />
+        </ThemeProvider>,
+      );
+
+      await act(async () => {
+        await Promise.resolve();
+      });
+
+      expect(screen.getByTestId("highlighted-code")).toHaveAttribute(
+        "data-theme",
+        "light",
+      );
+      expect(observer).not.toHaveBeenCalled();
+    } finally {
+      vi.stubGlobal("MutationObserver", originalMutationObserver);
+    }
+  });
+});