refactor(ui): extract auth and avatar components

src/app/chat/components/message-avatar.tsx

@@ -1,16 +1,7 @@
 "use client";
-/**
- * MessageAvatar 消息头像
- *
- * 原始 Dart: lib/ui/chat/widgets/message_avatar.dart（85 行）
- *
- * 显示规则：
- *   - AI 消息：Elio 头像（`/images/chat/pic-chat-elio.png`）
- *   - 用户消息（有 avatarUrl）：用户头像
- *   - 用户消息（无 avatarUrl）：游客头像（`/images/chat/pic-chat-guest.png`）
- */
 import Image from "next/image";
 
+import { UserMessageAvatar } from "./user-message-avatar";
 import styles from "./message-avatar.module.css";
 
 export interface MessageAvatarProps {
@@ -33,22 +24,5 @@ export function MessageAvatar({ isFromAI, userAvatarUrl }: MessageAvatarProps) {
     );
   }
 
-  if (userAvatarUrl && userAvatarUrl.length > 0) {
-    return (
-      <div className={styles.avatar} aria-label="User avatar">
-        <Image src={userAvatarUrl} alt="" width={43} height={43} />
-      </div>
-    );
-  }
-
-  return (
-    <div className={styles.avatar} aria-label="Guest avatar">
-      <Image
-        src="/images/chat/pic-chat-guest.png"
-        alt="Guest"
-        width={43}
-        height={43}
-      />
-    </div>
-  );
+  return <UserMessageAvatar avatarUrl={userAvatarUrl} />;
 }