Reduce first-screen load time by deferring non-critical startup work

[Copilot]

用户反馈首页加载体感明显偏慢（可达约 10 秒），目标是将首屏可用时间压到 5 秒以内。此 PR 聚焦首屏阻塞路径，减少初始化阶段的主线程竞争与视觉阻塞时长。

• 启动路径瘦身（frontend/src/main.js）

  • 移除重复的 Bootstrap JS 引入，避免重复执行与不必要体积开销。
  • 将非关键第三方初始化（GA / Clarity / Chatwoot / Sentry）从应用启动阶段后移到 window.load 后执行，并优先走 requestIdleCallback，无该能力时使用短延迟回退。
  • 将第三方初始化与 Service Worker 更新提示挂载收敛到同一 load 生命周期，避免多处并行监听导致的初始化时序噪音。

• 首访遮罩时长优化（frontend/src/components/BookSplash.vue）

  • 缩短首访开屏动画总时长（3200ms -> 1200ms），减少首页内容被遮挡时间。
  • 同步调整品牌文案出现时机（1000ms -> 700ms），确保开屏节奏更紧凑。

• 关键变更示例

window.addEventListener("load", () => {
  const runThirdPartyInit = () => {
    initThirdPartyServices().catch((error) => {
      console.error("[3P] 初始化失败:", error);
    });
  };

  if ("requestIdleCallback" in window) {
    window.requestIdleCallback(() => runThirdPartyInit());
  } else {
    setTimeout(() => runThirdPartyInit(), 150);
  }
});

[gitguardian]

⚠️ GitGuardian has uncovered 1 secret following the scan of your pull request.

Please consider investigating the findings and remediating the incidents. Failure to do so may lead to compromising the associated services or software components.

🔎 Detected hardcoded secret in your pull request

GitGuardian id	GitGuardian status	Secret	Commit	Filename
23769573	Triggered	Generic High Entropy Secret	fa6307b	frontend/src/main.js	View secret

🛠 Guidelines to remediate hardcoded secrets

1. Understand the implications of revoking this secret by investigating where it is used in your code.
2. Replace and store your secret safely. Learn here the best practices.
3. Revoke and rotate this secret.
4. If possible, rewrite git history. Rewriting git history is not a trivial act. You might completely break other contributing developers' workflow and you risk accidentally deleting legitimate data.

To avoid such incidents in the future consider

• following these best practices for managing and storing secrets including API keys and other credentials
• install secret detection on pre-commit to catch secret before it leaves your machine and ease remediation.

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^{🦉 GitGuardian detects secrets in your source code to help developers and security teams secure the modern development process. You are seeing this because you or someone else with access to this repository has authorized GitGuardian to scan your pull request.}