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/services/upload.go

@@ -39,6 +39,7 @@ type UploadService struct {
 
 type UploadOptions struct {
 	MaxDays           int
+	ExpiresInMinutes  int
 	MaxDownloads      int
 	Password          string
 	ObfuscateMetadata bool
@@ -199,14 +200,20 @@ func (s *UploadService) CreateBox(files []*multipart.FileHeader, opts UploadOpti
 		opts.MaxDays = 7
 	}
 
+	now := time.Now().UTC()
+	expiresAt := now.Add(time.Duration(opts.MaxDays) * 24 * time.Hour)
+	if opts.ExpiresInMinutes > 0 {
+		expiresAt = now.Add(time.Duration(opts.ExpiresInMinutes) * time.Minute)
+	}
+
 	box := Box{
 		ID:               randomID(10),
 		OwnerID:          strings.TrimSpace(opts.OwnerID),
 		CollectionID:     strings.TrimSpace(opts.CollectionID),
 		CreatorIP:        strings.TrimSpace(opts.CreatorIP),
 		StorageBackendID: normalizeBackendID(opts.StorageBackendID),
-		CreatedAt:        time.Now().UTC(),
-		ExpiresAt:        time.Now().UTC().Add(time.Duration(opts.MaxDays) * 24 * time.Hour),
+		CreatedAt:        now,
+		ExpiresAt:        expiresAt,
 		MaxDownloads:     opts.MaxDownloads,
 		Obfuscate:        opts.ObfuscateMetadata && strings.TrimSpace(opts.Password) != "",
 		Files:            make([]File, 0, len(files)),