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;
 }

backend/static/css/30-download.css

@@ -678,7 +678,54 @@
   color: var(--muted-foreground);
 }
 
-.file-emblem svg {
+.svg-icon {
+  width: 1rem;
+  height: 1rem;
+  display: inline-block;
+  flex: 0 0 auto;
+  background-color: currentColor;
+  vertical-align: -0.125em;
+  mask: var(--svg-icon-url) center / contain no-repeat;
+  -webkit-mask: var(--svg-icon-url) center / contain no-repeat;
+}
+
+.svg-icon-document {
+  --svg-icon-url: url("/static/icons/regular/submit-document.svg");
+}
+
+.svg-icon-share {
+  --svg-icon-url: url("/static/icons/regular/share-android.svg");
+}
+
+.svg-icon-download {
+  --svg-icon-url: url("/static/icons/regular/download.svg");
+}
+
+.svg-icon-list {
+  --svg-icon-url: url("/static/icons/regular/list.svg");
+}
+
+.svg-icon-grid {
+  --svg-icon-url: url("/static/icons/regular/view-grid.svg");
+}
+
+.svg-icon-emoji {
+  --svg-icon-url: url("/static/icons/regular/emoji.svg");
+}
+
+.svg-icon-open {
+  --svg-icon-url: url("/static/icons/regular/open-in-browser.svg");
+}
+
+.svg-icon-copy {
+  --svg-icon-url: url("/static/icons/regular/copy.svg");
+}
+
+.svg-icon-eye {
+  --svg-icon-url: url("/static/icons/regular/eye.svg");
+}
+
+.file-emblem .svg-icon {
   width: 1.75rem;
   height: 1.75rem;
 }
@@ -828,7 +875,7 @@
   justify-content: center;
 }
 
-.view-toolbar svg {
+.view-toolbar .svg-icon {
   width: 0.95rem;
   height: 0.95rem;
 }
@@ -997,14 +1044,9 @@
   pointer-events: auto;
 }
 
-.reaction-button svg {
+.reaction-button .svg-icon {
   width: 1.15rem;
   height: 1.15rem;
-  fill: none;
-  stroke: currentColor;
-  stroke-width: 1.9;
-  stroke-linecap: round;
-  stroke-linejoin: round;
 }
 
 .file-card:hover .reaction-button,