Cancel drag-and-drop with the Esc key (#1301)

[mstijak]

Closes #1301.

Summary

Pressing Esc during a drag-and-drop operation now cancels it: nothing is dropped, drop zones reset their state, the drag clone is removed, and the mouse/touch capture is released.

Changes

drag-drop/ops.tsx

• A global keydown listener is registered for the duration of a drag. On Escape it runs the end sequence via notifyDragDrop(null, /*cancelled*/ true) — skipping onDrop — and tears down the capture.
• Introduces a dedicated DragEndEvent with a cancelled flag, so onDragEnd handlers can distinguish a cancel from a drop (and still read result). On a keyboard cancel there is no pointer event, so its event and cursor are null.

overlay/captureMouse.ts

• The capture helpers (captureMouse/captureMouseOrTouch and the *2 variants) now return a teardown function so the capture can be released programmatically. Additive — existing callers ignore the return value.

DropZone / DragSource / Grid

• onDragEnd now receives a DragEndEvent. onDrop and onDragAway continue to use DragEvent (they only fire on a real pointer drop).

Behavior notes

• The normal drop path is unchanged: onDrop / onDragAway / onDragEnd fire as before with cancelled: false.
• On cancel, onDrop and the end-of-drag onDragAway do not fire (no pointer event); onDragEnd fires for every started zone, which fully resets zone state, with cancelled: true.

Tests

yarn check-types clean, yarn test 525 passing. Drag-and-drop is event/DOM-driven with no unit coverage, so this was verified by type-checking and manual reasoning through both the drop and cancel paths.

Notes

• Changelog intentionally omitted — to be added with the release.