feat(handlers): add file icons with standard and retro variants

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.

backend/static/file-icons/standard/word-document-svgrepo-com.svg

@@ -0,0 +1,10 @@
+<?xml version="1.0" encoding="utf-8"?>
+
+<!-- Uploaded to: SVG Repo, www.svgrepo.com, Generator: SVG Repo Mixer Tools -->
+<svg width="800px" height="800px" viewBox="-4 0 64 64" xmlns="http://www.w3.org/2000/svg">
+
+<g fill-rule="evenodd">
+
+<path d="m5.11 0a5.07 5.07 0 0 0 -5.11 5v53.88a5.07 5.07 0 0 0 5.11 5.12h45.78a5.07 5.07 0 0 0 5.11-5.12v-38.6l-18.94-20.28z" fill="#107cad"/>
+
+<path d="m56 20.35v1h-12.82s-6.31-1.26-6.13-6.71c0 0 .21 5.71 6 5.71z" fill="#084968"/>