premiumizeme: fix server side directory move after API changes

Apparently moving a directory using the id "0" as the root no longer
works, so this reads the real root ID when it is listed and uses that.

This fixes the DirMove problem.

See: https://forum.rclone.org/t/premiumize-cant-move-files/27169
See: #5734

backend/premiumizeme/api/types.go

@@ -58,6 +58,7 @@ type FolderListResponse struct {
 	Content  []Item `json:"content"`
 	Name     string `json:"name,omitempty"`
 	ParentID string `json:"parent_id,omitempty"`
+	FolderID string `json:"folder_id,omitempty"`
 }
 
 // FolderCreateResponse is the response to folder/create