feat: migrate from Create React App to Vite

Rebuild the site on the stack the other converted sites share: Vite 8,
React 19, TypeScript 7, react-router 8, Biome and sanitize.css, with the
site data moved out of the bundle into static-contents/.

- base the database code on cerebellum.neuroinf.jp and keep the pupil
  platform's table layout, about page and menu
- load the Flickr badge feed as JSONP without extra libraries
- serve the XooNIps data from modules/xoonips/, redirecting the old
  download and /database/file URLs
- restate the column widths as border-box totals and keep form controls
  on white
- track page views with GA4

src/common/MainContent.module.css

@@ -16,4 +16,4 @@
 
 .mainContent a:hover {
     color: #ff9900;
-}
+}