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/pages/api.html

@@ -85,6 +85,7 @@
           <span><code>file</code></span><p>One or more files for curl, browser, and generic multipart clients.</p>
           <span><code>sharex</code></span><p>One or more files from ShareX custom uploader configs.</p>
           <span><code>max_days</code></span><p>Optional number of days before expiration. Defaults to 7.</p>
+          <span><code>expires_minutes</code></span><p>Optional lifetime in minutes. Takes precedence over <code>max_days</code> when greater than zero — useful for sub-day expiries (e.g. <code>60</code> for one hour).</p>
           <span><code>max_downloads</code></span><p>Optional download count limit.</p>
           <span><code>password</code></span><p>Optional password required before viewing/downloading.</p>
           <span><code>obfuscate_metadata</code></span><p>Optional <code>on</code>; hides names/counts until unlock when a password is set.</p>