[Bug]: No bulk-clear for labels, and Ctrl+right-drag applies the current class instead of clearing

[airshiner]

Summary

The labeler's interaction grammar is inconsistent. Right-click clears a single label and Ctrl+drag box-labels, but Ctrl+right-drag box-labels rather than box-clearing. There is no bulk-clear at all.

Steps to reproduce

1. Set the class to Damaged.
2. Ctrl+right-drag a box over a group of unlabeled buildings.
3. They are labeled Damaged — the current class is applied rather than the selection cleared.

Expected

If right-click clears a label, Ctrl+right-drag should clear the selection, mirroring Ctrl+drag = box-label.

Impact

Two problems:

Effort. Clearing labels is a normal part of iterating on a model, particularly when rebalancing classes. The linear probe is very sensitive to the intact:damaged ratio, so removing redundant negatives is a legitimate and sometimes necessary tuning step. With no bulk-clear, removing ~2,400 redundant labels required ~2,400 individual right-clicks.

Data quality. An accidental Ctrl+right-drag silently applies the current class to every building in the box. I did this by accident and mislabeled a city building as Damaged without noticing at the time.

Suggested fix

Implement Ctrl+right-drag as box-clear, consistent with right-click = clear.