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/90-responsive.css

@@ -11,6 +11,10 @@
     padding: 2rem 0;
   }
 
+  .upload-grid {
+    grid-template-columns: 1fr;
+  }
+
   .option-grid,
   .form-footer,
   .result-header,