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/css/20-upload.css

@@ -13,6 +13,21 @@
   text-align: center;
 }
 
+.hero-eyebrow {
+  margin: 0 0 2.5rem 0;
+  display: inline-flex;
+  align-items: center;
+  gap: 0.4rem;
+  border-radius: 999px;
+  padding: 0.3rem 0.85rem;
+  background: var(--surface-1);
+  border: 1px solid var(--border);
+  color: var(--muted-foreground);
+  font-size: 0.76rem;
+  font-weight: 600;
+  letter-spacing: 0.01em;
+}
+
 
 .drop-zone {
   min-height: 19rem;
@@ -23,7 +38,7 @@
   padding: 2rem;
   border: 2px dashed var(--border);
   border-radius: var(--radius);
-  background: rgba(39, 39, 42, 0.42);
+  background: var(--surface-1);
   text-align: center;
   cursor: pointer;
   transition: border-color 160ms ease, background 160ms ease;
@@ -32,7 +47,7 @@
 .drop-zone:hover,
 .drop-zone.is-dragging {
   border-color: var(--primary);
-  background: rgba(39, 39, 42, 0.68);
+  background: var(--surface-1-hover);
 }
 
 .drop-zone input {
@@ -76,7 +91,7 @@
   margin-top: 1rem;
   border: 1px solid var(--border);
   border-radius: var(--radius);
-  background: rgba(39, 39, 42, 0.28);
+  background: var(--surface-2);
   padding: 0.75rem 0.9rem;
 }
 
@@ -140,7 +155,7 @@ button {
 }
 
 .button-primary:hover {
-  background: #e4e4e7;
+  background: var(--primary-hover);
 }
 
 .button-outline {