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/image-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" clip-rule="evenodd">
+
+<path d="M5.125.042c-2.801 0-5.072 2.273-5.072 5.074v53.841c0 2.803 2.271 5.073 5.072 5.073h45.775c2.801 0 5.074-2.271 5.074-5.073v-38.604l-18.904-20.311h-31.945z" fill="#49C9A7"/>
+
+<path d="M55.977 20.352v1h-12.799s-6.312-1.26-6.129-6.707c0 0 .208 5.707 6.004 5.707h12.924z" fill="#37BB91"/>