refactor(download): migrate inline SVGs to CSS mask-based icons

Replaces inline SVG elements in the download template with a reusable
CSS mask-based icon system. This reduces HTML bloat and centralizes
icon management.

- Added a generic `.svg-icon` utility class using CSS masks.
- Defined specific icon classes mapping to static SVG assets.
- Updated `download.html` to use `<span>` tags with the new icon classes.
- Adjusted CSS selectors in retro and download stylesheets to target `.svg-icon`.

backend/static/css/16-retro.css

@@ -655,7 +655,7 @@
   padding: 0;
 }
 
-:root[data-theme="retro"] .view-toolbar .icon-button svg {
+:root[data-theme="retro"] .view-toolbar .icon-button .svg-icon {
   margin: 0;
   display: block;
 }