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

2. a11y audit, e.g. 放大 200%, keyboard navigation, image modal, image modal file input error live region 似乎不太穩定

3. refactor：e.g. tailwind 4 的倍數

4. 自動測試