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/templates/layouts/base.html

@@ -15,8 +15,10 @@
   {{if .ImageURL}}<meta property="og:image" content="{{.ImageURL}}">{{end}}
   <meta name="twitter:card" content="summary_large_image">
   {{if .ImageURL}}<meta name="twitter:image" content="{{.ImageURL}}">{{end}}
+  <script src="/static/js/05-theme.js"></script>
   <link rel="stylesheet" href="/static/css/00-base.css">
   <link rel="stylesheet" href="/static/css/10-layout.css">
+  <link rel="stylesheet" href="/static/css/15-revamp.css">
   <link rel="stylesheet" href="/static/css/20-upload.css">
   <link rel="stylesheet" href="/static/css/30-download.css">
   <link rel="stylesheet" href="/static/css/40-docs.css">
@@ -58,6 +60,13 @@
 
   <footer class="site-footer">
     <span>{{.AppName}} · {{.CurrentYear}} · self-hosted</span>
+    <label class="theme-picker">
+      <span>Theme</span>
+      <select data-theme-select aria-label="Site theme">
+        <option value="revamp">Aurora (default)</option>
+        <option value="classic">Classic</option>
+      </select>
+    </label>
     <span class="footer-links">{{if .CurrentUser}}<a href="/app">Dashboard</a><a href="/api">API</a><a href="/account/settings">Account</a>{{else}}<a href="/login">Sign in</a><a href="/api">API</a>{{end}}</span>
   </footer>
 </body>