fix: axis drag stops when cursor leaves the axis widget bounding box

[dylansproule]

Fixes #794

Problem

Dragging the X/Y axis to scroll or zoom stops as soon as the cursor moves outside the widget's bounding box, because the drag handlers are driven by _findWidgetByEvent in Event.ts, which only resolves a widget when the pointer is over it.

Change

Moved the axis drag scaling logic out of Event.ts into the axis widgets themselves (mirroring the existing SeparatorWidget pattern):

• YAxisWidget now owns Y-axis drag scaling, manual-axis range sync, and the double-click reset (zoomYAxis / resetYAxisAndManualYAxes), registering mouseDownEvent / pressedMouseMoveEvent / mouseUpEvent handlers.
• XAxisWidget now owns X-axis zoom-on-drag with the same handlers.
• Event.ts routes X_AXIS/Y_AXIS mouse and touch down/move/up events through this._mouseDownWidget so the drag target is fixed at pointer-down and the handlers continue firing regardless of where the cursor is.
• mouseWheelVertEvent and mouseDoubleClickEvent now delegate to the widget public methods.

Notes

• _processMainScrollingEvent stays in Event.ts (MAIN widget behavior unchanged).
• Lint and type-check pass.