Introduce file icon support to the file browser. Icons are loaded on
startup and mapped based on file name and content type.
- Load file icon mappings in the App handler initialization.
- Add `HasThumbnail`, `IconURL`, and `IconRetroURL` to the file view.
- Update CSS to support displaying file icons alongside thumbnails.
- Add retro theme support to swap standard icons with pixelated retro
variants when the retro theme is active.
