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/libs/handlers/upload.go

@@ -79,8 +79,14 @@ func (a *App) Upload(w http.ResponseWriter, r *http.Request) {
 		helpers.WriteJSONError(w, http.StatusRequestEntityTooLarge, fmt.Sprintf("expiration cannot exceed %d days", effectivePolicy.MaxDays))
 		return
 	}
+	expiresMinutes := parseInt(r.FormValue("expires_minutes"))
+	if expiresMinutes > 0 && !isAdminUpload && expiresMinutes > effectivePolicy.MaxDays*24*60 {
+		helpers.WriteJSONError(w, http.StatusRequestEntityTooLarge, fmt.Sprintf("expiration cannot exceed %d days", effectivePolicy.MaxDays))
+		return
+	}
 	result, err := a.uploadService.CreateBox(files, services.UploadOptions{
 		MaxDays:           maxDays,
+		ExpiresInMinutes:  expiresMinutes,
 		MaxDownloads:      parseInt(r.FormValue("max_downloads")),
 		Password:          r.FormValue("password"),
 		ObfuscateMetadata: r.FormValue("obfuscate_metadata") == "on",