perf: defer heavy modules and add performance baselines

Lazy-load Stripe and the chat reply animation, remove the unused Lottie runtime, and add repeatable bundle and Web Vitals baseline tooling.

src/app/chat/components/chat-area.tsx

@@ -12,7 +12,14 @@
  *   - 新消息到达时自动滚到底部
  *   - 用户向上滚动时不强制拉回（保持阅读上下文）
  */
-import { useEffect, useLayoutEffect, useMemo, useRef } from "react";
+import {
+  lazy,
+  Suspense,
+  useEffect,
+  useLayoutEffect,
+  useMemo,
+  useRef,
+} from "react";
 
 import type { UiMessage } from "@/data/dto/chat";
 
@@ -23,11 +30,15 @@ import {
 } from "../chat-render-items";
 import { AiDisclosureBanner } from "./ai-disclosure-banner";
 import { DateHeader } from "./date-header";
-import { LottieMessageBubble } from "./lottie-message-bubble";
 import { MessageBubble } from "./message-bubble";
 import styles from "./chat-area.module.css";
 
 const CHAT_AUTO_SCROLL_BOTTOM_THRESHOLD = 96;
+const ReplyingAnimation = lazy(() =>
+  import("./lottie-message-bubble").then((module) => ({
+    default: module.LottieMessageBubble,
+  })),
+);
 
 export interface ChatAreaProps {
   messages: readonly UiMessage[];
@@ -104,7 +115,11 @@ export function ChatArea({
         onOpenImage,
       )}
 
-      {isReplyingAI && <LottieMessageBubble />}
+      {isReplyingAI ? (
+        <Suspense fallback={null}>
+          <ReplyingAnimation />
+        </Suspense>
+      ) : null}
     </main>
   );
 }