fix: compute axis drag zoom from local coordinates

[NemeZZiZZ]

Problem

Axis drag-zoom computes its scale from absolute page coordinates while every other gesture in Event.ts uses widget-local event.x/event.y:

this._xAxisStartScaleDistance = event.pageX          // :746
const scale = this._xAxisStartScaleDistance / event.pageX   // :755
...
this._yAxisStartScaleDistance = event.pageY          // :776
const scaleFactor = event.pageY / this._yAxisStartScaleDistance   // :788

Consequences:

1. Sensitivity depends on where the chart sits on the page. A 50px drag starting at pageX=500 yields scale 500/550 ≈ 0.91; the same drag starting at pageX=50 yields 0.5 — ten times the zoom for the same physical movement.
2. Page scroll mid-drag shifts pageX/pageY and the scale jumps.
3. 0 doubles as the "not set" sentinel (!== 0), so a drag starting at page coordinate 0 — a full-width chart flush against the left/top edge of the page — silently disables axis zoom entirely.

Fix

• Use widget-local event.x / event.y (already what _xAxisStartScaleCoordinate stores on the previous line).
• Replace the 0 sentinel with null (Nullable<number>), matching _xAxisStartScaleCoordinate.
• Guard the two newly reachable degenerate cases that page coordinates made practically impossible: a drag starting on the exact first pixel of the widget (start === 0, ratio meaningless) and the pointer reaching the widget's first pixel mid-drag (event.x/y === 0 → division by zero / zero scale factor). Both now skip that event instead of producing Infinity or collapsing the range to a point.

The hyperbolic response curve (start/current) is intentionally unchanged — this PR removes the external position/scroll dependency, it does not redesign the gesture feel.