fix: check path existence

[BLumia]

Fix #54

from: QStandardPaths::writableLocation:

Note: The storage location returned can be a directory that does not exist; i.e., it may need to be created by the system or the user.

For linux, QStandardPath read the ~/.config/user-dirs.dirs to get the QStandardPaths::DesktopLocation path and the returned path may not exist, so we need manually check / create the path first.