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/20-upload.css

@@ -1,5 +1,5 @@
 .upload-view {
-  width: min(48rem, calc(100% - 2rem));
+  width: min(64rem, calc(100% - 2rem));
   min-height: calc(100vh - 7.25rem);
   margin: 0 auto;
   padding: 2.5rem 0;
@@ -9,6 +9,45 @@
   gap: 1.5rem;
 }
 
+/* Two-column upload layout: drop-zone window on the left, options on the
+   right. Collapses to a single column on narrow screens (see 90-responsive). */
+.upload-grid {
+  display: grid;
+  grid-template-columns: minmax(0, 3fr) minmax(0, 2fr);
+  gap: 1.25rem;
+  align-items: start;
+}
+
+.upload-main,
+.upload-options {
+  margin: 0;
+}
+
+.options-title {
+  margin: 0 0 1.1rem;
+  font-size: 1.05rem;
+  font-weight: 650;
+  line-height: 1.2;
+}
+
+/* Stack the option fields vertically in the narrower right-hand window. */
+.upload-options .option-grid {
+  grid-template-columns: 1fr;
+  margin-top: 0;
+}
+
+/* Summary + upload button sit at the bottom of the options window. */
+.upload-options .form-footer {
+  flex-direction: column;
+  align-items: stretch;
+  gap: 0.75rem;
+  margin-top: 1.25rem;
+}
+
+.upload-options .form-footer .button {
+  width: 100%;
+}
+
 .hero-copy {
   text-align: center;
 }