Improve consistency between Central (Frontend) and Figma

We have an opportunity to go through the Frontend code and make a more explicit link between styles in the code and styles in [Figma](https://www.figma.com/design/ULpCuTKkIys6Y99YGyMcrF/Central?node-id=36-615&p=f&m=dev).

This isn't critical for the 2025.1 release but it would be nice!

Some examples (mainly involving headers and text size)
* the page [heading](https://github.com/getodk/central-frontend/pull/1158/files#diff-ee68b5c0bca99e0dd0a4a07bfac565652e2051befefb2c6f5a5133ffc070c11eR171) on the submission/entity details page was made slightly smaller but that change can be made across all of Central
* We could use more header-based sizing instead of hardcoding font sizes
* There are pages with many different sizes of fonts and more pieces could use the same font sizes (e.g. an empty project description could be all the same size and not italic.)