feat(upload): add dynamic expiry options and modern UI theme

- Implement dynamic expiry options on the upload page based on user roles and retention policies.
- Add helper functions to build and format expiry options into human-readable labels.
- Introduce a new modern theme featuring glassmorphism, gradients, and frosted glass cards.

backend/static/js/40-upload.js

@@ -18,6 +18,28 @@
     return;
   }
 
+  // Remember the last-chosen expiry across uploads (per browser).
+  const expirySelect = form.querySelector("[data-expiry-select]");
+  if (expirySelect) {
+    const EXPIRY_KEY = "warpbox-expiry";
+    let saved = null;
+    try {
+      saved = localStorage.getItem(EXPIRY_KEY);
+    } catch (e) {
+      saved = null;
+    }
+    if (saved && expirySelect.querySelector('option[value="' + saved + '"]')) {
+      expirySelect.value = saved;
+    }
+    expirySelect.addEventListener("change", () => {
+      try {
+        localStorage.setItem(EXPIRY_KEY, expirySelect.value);
+      } catch (e) {
+        /* ignore persistence failures */
+      }
+    });
+  }
+
   let latestBoxURL = "";
   let selectedFiles = [];