feat(upload): add pause and cancel controls for active uploads

- Add CSS grid layout for upload-active-actions and hidden state
- Implement JavaScript logic for pausing and cancelling uploads with confirmation
- Add test to verify home page includes upload control elements

backend/static/css/20-upload.css

@@ -48,6 +48,16 @@
   width: 100%;
 }
 
+.upload-active-actions {
+  display: grid;
+  grid-template-columns: 1fr 1fr;
+  gap: 0.75rem;
+}
+
+.upload-active-actions[hidden] {
+  display: none !important;
+}
+
 .upload-options .form-footer .upload-new-button {
   margin-top: -0.25rem;
 }