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/thread-messages.test.tsx

@@ -1,7 +1,11 @@
 import { render, screen } from "@testing-library/react";
 import { describe, expect, it } from "vitest";
 
-import { ThreadMessages } from "@/components/thread/ThreadMessages";
+import {
+  assistantCopyFlags,
+  buildDisplayUnits,
+  ThreadMessages,
+} from "@/components/thread/ThreadMessages";
 import type { UIMessage } from "@/lib/types";
 
 describe("ThreadMessages", () => {
@@ -89,4 +93,37 @@ describe("ThreadMessages", () => {
     render(<ThreadMessages messages={messages} isStreaming={false} />);
     expect(screen.getAllByRole("button", { name: "Copy reply" })).toHaveLength(1);
   });
+
+  it("computes final assistant copy flags with user-boundary semantics", () => {
+    const units = buildDisplayUnits([
+      { id: "u1", role: "user", content: "one", createdAt: 1 },
+      { id: "a1", role: "assistant", content: "draft", createdAt: 2 },
+      {
+        id: "t1",
+        role: "tool",
+        kind: "trace",
+        content: "tool()",
+        traces: ["tool()"],
+        createdAt: 3,
+      },
+      { id: "a2", role: "assistant", content: "final", createdAt: 4 },
+      { id: "u2", role: "user", content: "two", createdAt: 5 },
+      { id: "a3", role: "assistant", content: "next", createdAt: 6 },
+    ]);
+
+    const flags = assistantCopyFlags(units);
+    const assistantFlags = units
+      .map((unit, index) =>
+        unit.type === "single" && unit.message.role === "assistant"
+          ? [unit.message.id, flags[index]]
+          : null,
+      )
+      .filter(Boolean);
+
+    expect(assistantFlags).toEqual([
+      ["a1", false],
+      ["a2", true],
+      ["a3", true],
+    ]);
+  });
 });