BUG: Deleted/missing folders still appear in Settings folder list

### Is there an existing issue for this?

- [x] I have searched the existing issues

### What happened?

### Is there an existing issue for this?

- [x] I have searched the existing issues

### What happened?

## Summary
Folders that are deleted from the filesystem still appear in **Settings → Folder Management** in PictoPy.

## Steps to reproduce
1. Add a folder in PictoPy (Settings → Folder Management → add folder).
2. Close PictoPy (optional).
3. Delete that folder from the local filesystem (Explorer/Finder).
4. Reopen PictoPy and go to Settings → Folder Management.

## Actual result
The deleted folder is still listed with its old path.

Here the first folder is deleted but it still showing in folder management

## Expected result
PictoPy should detect missing folders and either:
- show them as **Missing** and let the user remove them from the library, or
- provide a **Clean up missing folders** / **Rescan** action.

## Notes
This is especially confusing because the folder path is shown as valid even when it no longer exists on disk.

### Record

- [x] I agree to follow this project's Code of Conduct

### Record

- [x] I agree to follow this project's Code of Conduct