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/_components/core/mobile-shell.module.css

@@ -5,7 +5,7 @@
   justify-content: center;
   width: 100%;
   min-height: 100dvh;
-  background: inherit;
+  background: #000000; /* 显式黑，桌面端 500px 外圈为黑色 */
 }
 
 .content {