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/txt-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">
+
+<path d="M5.151-.036c-2.803 0-5.074 2.272-5.074 5.074v53.841c0 2.803 2.271 5.074 5.074 5.074h45.774c2.801 0 5.074-2.271 5.074-5.074v-38.605l-18.902-20.31h-31.946z" fill-rule="evenodd" clip-rule="evenodd" fill="#F9CA06"/>
+
+<g fill-rule="evenodd" clip-rule="evenodd">
+
+<path d="M56.008 20.316v1h-12.799s-6.312-1.26-6.129-6.708c0 0 .208 5.708 6.004 5.708h12.924z" fill="#F7BC04"/>