test(charts): the pie lays out at 600 and 1600 px (#1160)

[ZhuchkaTriplesix]

Refs #1160

Changes

• Widget tests: the pie lays out without exceptions at 600 px and at 1600 px wide.

Already on dev before this PR

• Radius from the pane size (LayoutBuilder), donut style.
• Legend beside the donut (wide) or below it (narrow), with hover linking legend row and slice.
• Default Top N for the pie (ChartData.pieTopN, Top 8 + Other); colours tinted from round to round so they stay distinct.
• Slice labels with contrast against the slice colour.
• The test for Top 8 with Other, distinct colours and a legend.

Not covered here

• The pie filling the pane is not measured; the test checks only for layout exceptions. The issue stays open until someone checks the size visually or with a size assertion.