feat(core): pointerdrag.js pointer-events based drag toolkit

[ralflang]

New Drag & Drop primitives library which can replace the prototypejs/scriptaculous based dragdrop2.js library.

#211 and horde/kronolith#76 together seem to fix horde/kronolith#74 (calendar event drag and drop move / resize) based on more modern javascript

[pierrefardel]

Tested together with horde/kronolith#76 — fixes horde/kronolith#74 (drag ghost now follows the cursor when scrolling mid-drag) and the resize issues. pointerdrag.js loads and works cleanly in month and day/week views. 👍 Details in horde/kronolith#76.

[TDannhauer]

Review

Looks good as a shared Pointer Events drag primitive. The design (thin toolkit, consumer-owned state, fixed-position ghosts, pointer capture, click-swallow after a real drag) is the right replacement path for dragdrop2 in apps that opt in.

Together with horde/kronolith#76 this addresses horde/kronolith#74 for month-view mid-drag scroll (viewport-fixed ghost + clientX/clientY).

Notes (non-blocking)

• Wheel/trackpad scroll mid-drag does not replay move handlers (only edge auto-scroll does). Month ghosts stay correct because they are position: fixed; consumers that derive layout from scrollTop may still want a scroll listener that replays the last pointer position.
• Consider touch-action: none on the source while a drag is pending/active so touch does not steal the gesture.
• releasePointerCapture only runs from destroy(); fine on normal pointerup, but a short comment would help future readers.

Approve. Do we need the Wheel/trackpad thing?

[ralflang]

Applied review feedback in 2f24cdf:

• touch-action: none on the source in the constructor (saved and restored in destroy).
• External-scroll replay: pointerdrag now listens on the drag's scroll container and on window and replays the last pointermove when either scrolls. Covers wheel, trackpad, kinetic touch, and JS-driven scroll uniformly with the existing auto-scroll replay path.
• Comment on why releasePointerCapture() only runs from destroy() (pointerup/pointercancel release implicitly per spec).