#3477372: Implement new design for top bar and sidebars

Introducing the new design language in the form of a new Panel component, which is now used for the top bar, sidebars, and the component insert panel. This was also a good opportunity to solidify how we work with and extend design tokens from Radix Themes, and to solidify our CSS file structure — all of which is now documented in the README.

The end-to-end test file changes are due a refactor the component insert panel has undergone: we're now using separate portals for the first and second level menus. Previously they were absolutely positioned within the same portal that made it impossible to nicely apply box-shadows in a way that was expected.