Split up css into smaller chunks

[JammingBen]

Currently, all our CSS gets merged into one big file because we've set cssCodeSplit: false in the vite config. The goal is to set this to true, as it would improve loading times in Web.

There is, however, one issue where some of the chunks won't be loaded, which leads so missing styles. Needs investigation why this happens. Lazy loading routes could help here, since chunks from lazy loaded things will definitely be loaded: #10980