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/audio-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="M5.151.012c-2.802 0-5.073 2.272-5.073 5.073v53.842c0 2.802 2.272 5.073 5.073 5.073h45.774c2.803 0 5.075-2.271 5.075-5.073v-38.606l-18.903-20.309h-31.946z" fill="#379FD3"/>
+
+<path d="M56 20.357v1h-12.8s-6.312-1.26-6.128-6.707c0 0 .208 5.707 6.003 5.707h12.925z" fill="#2987C8"/>