v2 收尾：refactor、tidy up

[wendyyuchensun]

1. 右欄 tooltip 不太好用：https://github.com/coseeing/Access8MathWeb/pull/129/changes#diff-5c940b3e659e90485d0d6f55e2336c022b70f7378a7ddac6ab5e30e0a93b52dcR39，a11y https://github.com/coseeing/Access8MathWeb/pull/129/changes#diff-5c940b3e659e90485d0d6f55e2336c022b70f7378a7ddac6ab5e30e0a93b52dcR39Add tabIndex={0} to the tooltip trigger span so keyboard users can access the alt-text tooltip.
   preview 顏色對比度
   preview 如果超出頁面位置
   考慮 image 位置
   preview 影響其他 aria describedby
   https://www.joshwcomeau.com/css/anchor-positioning/

---

這區問題，要先決定 template 是不是也要同步：

3. external link aria describededby 3af6870

4. 右欄 darkmode 可否用 tailwind 處理？

module.exports = {    darkMode: ['variant', '.right-side-preview-area.darkmode &'],
    content: ['./src/**/*.{js,jsx,ts,tsx}'],
    // ...
  }

9. 連動匯出的 template 是否要更新 CSS？

---

12. review code & refactor