feat(archive): add retro theme support to archive browser

Implement retro-themed styling and classic pixelated icons for the
archive browser when the "retro" theme is active.

Changes include:
- Adding CSS overrides for `[data-theme="retro"]` to style the archive
  browser container, tree nodes, and hover states.
- Updating the JS preview script to dynamically append retro image
  icons (e.g., classic shell32/zipfldr icons) alongside SVGs.
- Toggling visibility between SVG and retro image icons using CSS
  based on the active theme.

backend/static/css/30-download.css

@@ -367,6 +367,20 @@
   color: var(--muted-foreground);
 }
 
+.archive-svg-icon,
+.archive-retro-icon {
+  display: inline-grid;
+  place-items: center;
+  width: 100%;
+  height: 100%;
+}
+
+.archive-retro-icon {
+  display: none;
+  object-fit: contain;
+  image-rendering: pixelated;
+}
+
 .archive-file-icon svg,
 .archive-chevron svg {
   fill: none;
@@ -408,6 +422,92 @@
   color: #f8fafc;
 }
 
+[data-theme="retro"] .archive-browser-preview {
+  border: 1px solid #000000;
+  background: #ffffff;
+  color: #000000;
+  box-shadow:
+    inset -1px -1px 0 #808080,
+    inset 1px 1px 0 #ffffff;
+}
+
+[data-theme="retro"] .archive-browser-header {
+  border-bottom: 1px solid #808080;
+  background: #c0c0c0;
+  color: #000000;
+  box-shadow:
+    inset -1px -1px 0 #808080,
+    inset 1px 1px 0 #ffffff;
+}
+
+[data-theme="retro"] .archive-browser-header span {
+  color: #404040;
+}
+
+[data-theme="retro"] .archive-tree {
+  background: #ffffff;
+  font-family: "PixelOperatorMono", "Courier New", monospace;
+}
+
+[data-theme="retro"] .archive-node-row {
+  min-height: 1.8rem;
+  border-radius: 0;
+  color: #000000;
+}
+
+[data-theme="retro"] .archive-node-row:hover {
+  background: #000078;
+  color: #ffffff;
+}
+
+[data-theme="retro"] .archive-node-size {
+  color: #404040;
+}
+
+[data-theme="retro"] .archive-node-row:hover .archive-node-size {
+  color: #ffffff;
+}
+
+[data-theme="retro"] .archive-chevron {
+  color: #000000;
+}
+
+[data-theme="retro"] .archive-folder[open] > summary .archive-chevron {
+  color: #000078;
+}
+
+[data-theme="retro"] .archive-node-row:hover .archive-chevron,
+[data-theme="retro"] .archive-node-row:hover .archive-file-icon {
+  color: #ffffff;
+}
+
+[data-theme="retro"] .archive-chevron svg {
+  width: 1.35rem;
+  height: 1.35rem;
+  stroke-width: 2.4;
+}
+
+[data-theme="retro"] .archive-file-icon {
+  width: 1.45rem;
+  height: 1.45rem;
+  color: #000000;
+}
+
+[data-theme="retro"] .archive-svg-icon {
+  display: none;
+}
+
+[data-theme="retro"] .archive-retro-icon {
+  display: block;
+}
+
+[data-theme="retro"] .archive-browser-empty,
+[data-theme="retro"] .archive-browser-legacy {
+  color: #000000;
+  background: #ffffff;
+  font-family: "PixelOperatorMono", "Courier New", monospace;
+}
+
 .archive-node-name {
   min-width: 0;
   overflow-wrap: anywhere;