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/character-intro.module.css

@@ -1,17 +0,0 @@
-/* CharacterIntro 角色介绍样式 */
-
-.intro {
-  padding: var(--spacing-2, 8px) var(--spacing-5, 20px);
-}
-
-.card {
-  padding: var(--spacing-3, 12px);
-  background: var(--color-bubble-background, rgba(255, 255, 255, 0.05));
-  border-radius: var(--radius-md, 8px);
-  font-size: var(--font-size-sm, 14px);
-  line-height: 1.5;
-  color: var(--color-text-secondary, #9e9e9e);
-  max-height: 7.5em; /* 5 行 max */
-  overflow: hidden;
-  backdrop-filter: blur(10px);
-}

src/app/chat/components/character-intro.tsx

@@ -1,40 +0,0 @@
-"use client";
-/**
- * CharacterIntro 角色介绍卡片
- *
- * 原始 Dart: lib/ui/chat/widgets/character_intro.dart（49 行）
- *
- * 视觉：模糊背景卡片，灰色字体，5 行 max
- * 用途：首次进入聊天时展示 AI 角色介绍（Elio 设定）
- */
-import styles from "./character-intro.module.css";
-
-export interface CharacterIntroProps {
-  /** 角色介绍文本 */
-  text?: string;
-  /** 最大行数（默认 5） */
-  maxLines?: number;
-}
-
-const DEFAULT_TEXT =
-  "Meet Elio Silvestri, your exclusive AI boyfriend. " +
-  "He's charming, witty, and always there for you. " +
-  "Chat with him anytime, anywhere — about your day, your dreams, or anything in between.";
-
-export function CharacterIntro({ text, maxLines = 5 }: CharacterIntroProps) {
-  return (
-    <div className={styles.intro} role="complementary" aria-label="Character intro">
-      <div
-        className={styles.card}
-        style={{
-          // 近似 -webkit-line-clamp（多行截断）
-          display: "-webkit-box",
-          WebkitLineClamp: maxLines,
-          WebkitBoxOrient: "vertical",
-        }}
-      >
-        {text ?? DEFAULT_TEXT}
-      </div>
-    </div>
-  );
-}

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";