Add traverse access level for folder navigation without file access

When a permission is set on a nested folder, traverse is automatically
granted on all ancestor folders so users can navigate to it. Traverse
only shows subfolders in listings — files are hidden. This prevents
orphaned permissions where a user has access to a nested folder but
can't reach it.

Hierarchy: traverse < view < edit < admin

packages/admin/src/components/storage/folder-permissions-dialog.tsx

@@ -28,6 +28,7 @@ interface FolderPermissionsDialogProps {
 }
 
 const ACCESS_LEVELS = [
+  { value: 'traverse', label: 'Traverse', variant: 'outline' as const },
   { value: 'view', label: 'View', variant: 'secondary' as const },
   { value: 'edit', label: 'Edit', variant: 'default' as const },
   { value: 'admin', label: 'Admin', variant: 'destructive' as const },