Fix NameError in ExportFolder.export progress signal

[KMneko]

In deepd3/core/export.py, ExportFolder.export() iterates over self.rois using for k, v in self.rois.items(), but emits the progress signal using int(z), where z is undefined:

self.zSignal.emit(int(z), len(self.rois.keys()))

This raises NameError: name 'z' is not defined on the first iteration. As a result, "Export ROIs to folder" only writes the first z-plane's ROIs instead of all planes.

This PR replaces z with k:

self.zSignal.emit(int(k), len(self.rois.keys()))

Tested locally (Python 3.9): after the change, "Export ROIs to folder" produces one folder per z-plane for the full stack.