Adding `graceful` stop function

[gnzng]

Summary

Implement a graceful stop function (likely via an additional button) that allows users to halt an in-progress scan while preserving the ability to export partial results through the bluesky-exporter.

Current Behavior

1. Scans run to completion or are forcibly terminated
2. Forced termination may leave data in an inconsistent state
3. No way to export partial scan results via bluesky-exporter into h5 standard files

Proposed Solution

Add a "Stop Scan" button that:

1. Signals the scanning process to halt at the next safe checkpoint
2. Allows currently processing items to complete
3. Ensures data remains in a consistent state for export
4. Maintains compatibility with existing bluesky-exporter functionality

Next steps

I will start looking into that.