fix: detach resize-handle document listeners on destroy before move threshold

[patrykskrzyniarz]

Summary

• Fixes a listener leak in DDResizableHandle.destroy(): document mousemove/mouseup listeners attached on mousedown were only removed when this.moving was true (after the 3px threshold).
• A handle destroyed between mousedown and that threshold left orphaned listeners that then threw Cannot read properties of undefined (reading 'gridstackNode') on every subsequent mouse event until reload.
• Guard on this.mouseDownEvent instead — same pattern as DDDraggable.destroy().

Fixes #3345

Test plan

• Init a resizable grid, mousedown on a .ui-resizable-handle without moving >3px, then call removeAll() / disable() / destroy() while still held
• Move the mouse — no gridstackNode TypeError, no repeating errors on later mouse events
• Normal resize still works (mousedown → drag past 3px → mouseup)
• Esc during an in-progress resize still cancels cleanly

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