feat(upload): add transfer rate tracking and 6-hour expiry option

- Implement real-time transfer rate tracking and display upload speed (e.g., Mb/s) in the progress status.
- Add a 6-hour (360 minutes) option to the upload expiry selection ladder.
- Fix an issue where the "new upload" button remained visible by explicitly toggling its display style and adding a CSS fallback for the `hidden` attribute.

backend/static/css/20-upload.css

@@ -52,6 +52,10 @@
   margin-top: -0.25rem;
 }
 
+.upload-options .form-footer .upload-new-button[hidden] {
+  display: none !important;
+}
+
 .hero-copy {
   text-align: center;
 }