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/chat/components/index.ts

@@ -32,5 +32,4 @@ export { PwaInstallDialog } from "./pwa-install-dialog";
 export { PwaInstallOverlay } from "./pwa-install-overlay";
 export { BrowserHintOverlay } from "./browser-hint-overlay";
 export { AiDisclosureBanner } from "./ai-disclosure-banner";
-export { CharacterIntro } from "./character-intro";
 export { LanguageDialog, DEFAULT_LANGUAGES } from "./language-dialog";