When the target extraction directory already exists, QDir::mkdir()
returns false and shows a misleading "Fix permissions" error.

Now shows a dialog with three options:
- Wipe & Extract: removes existing directory and proceeds
- Create New Folder: appends (1), (2), etc. to find an available name
- Cancel: aborts extraction

… directory suffix to 100

Assign QFileDialog result to a local variable before checking for cancel,
so backupFilename is preserved when the user cancels the open dialog after
a drag-and-drop. Also cap the "Create New Folder" suffix loop at 100.