chore: cleanup unused exports and fix mobile shell background

- Remove unused CharacterIntro export from chat components index
- Drop stale JSDoc block from splash-screen server component
- Set mobile shell background to explicit black so the desktop area outside the 500px viewport is black instead of inheriting from parent

src/app/splash/components/splash-screen.tsx

@@ -1,13 +1,3 @@
-/**
- * Splash 屏幕（纯布局 / Server Component）
- *
- * 鉴权流程逻辑已封装到 `<SplashButton>` 内部（client 组件）：
- *   - 已登录检测 / 登录成功跳转 / Facebook 登录 dispatch
- *   - 跨 store 初始化（ChatAuthStatusChanged / UserInit）
- *
- * 本组件职责：纯 JSX 布局，无 hooks → 可作为 Server Component 直接 SSR。
- */
-
 import { MobileShell } from "@/app/_components/core/mobile-shell";
 
 import { SplashBackground } from "./splash-background";