Side Nav
The side nav is the left column of the app.

It holds three parts, top to bottom:
- The page list: one link per page in the App. The link of the current page is highlighted, and a page's emoji is shown in front of its title.
- The page's Sidebar container, when the page func puts anything in it.
- The app controls:
- Rerun: Rerun the Page Func without changing any state.
- Dark/Light Mode Switch: Switch the theme of the app.
- A spinner, shown while the app is running the Page Func.
On a narrow screen the column collapses behind a Menu button.