feat(ui): redesign upload page into a two-column layout

Redesigns the upload interface to use a two-column grid layout on larger screens, separating the file drop-zone (left) from the upload options (right). This improves usability and visual hierarchy.

Changes include:
- Increasing the upload view max-width to 64rem.
- Creating a responsive `.upload-grid` that collapses to a single column on narrow viewports.
- Stacking option fields vertically in the narrower options panel.
- Adding retro theme support for the new options title.

backend/static/css/16-retro.css

@@ -257,7 +257,8 @@
   margin-bottom: 0;
 }
 
-:root[data-theme="retro"] .docs-card h2 {
+:root[data-theme="retro"] .docs-card h2,
+:root[data-theme="retro"] .upload-options .options-title {
   display: flex;
   align-items: center;
   justify-content: space-between;
@@ -279,7 +280,8 @@
   margin: -1.5rem -1.5rem 1rem;
 }
 
-:root[data-theme="retro"] .docs-card h2::after {
+:root[data-theme="retro"] .docs-card h2::after,
+:root[data-theme="retro"] .upload-options .options-title::after {
   content: "✕";
   display: grid;
   place-items: center;