Implement retro-themed styling and classic pixelated icons for the
archive browser when the "retro" theme is active.
Changes include:
- Adding CSS overrides for `[data-theme="retro"]` to style the archive
browser container, tree nodes, and hover states.
- Updating the JS preview script to dynamically append retro image
icons (e.g., classic shell32/zipfldr icons) alongside SVGs.
- Toggling visibility between SVG and retro image icons using CSS
based on the active theme.
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.
