fix(legend): use bottom:0 default so legend no longer overlaps axis labels

[waterWang]

Problem

When the chart has a grid with a bottom offset (or containLabel: true), the legend overlaps with the x-axis labels. This is because the default legend position is bottom: tokens.size.m (15px from the container bottom), which places the legend too close to the grid's bottom edge where axis labels are rendered.

Steps to Reproduce

1. Create a stacked line chart with grid: { bottom: 80 } (or any explicit bottom value)
2. Observe the legend overlapping with the x-axis labels

Current Behavior

Legend overlaps axis labels when the grid has a bottom offset.

Workaround

legend: {
  bottom: 0,
  padding: [0, 0, 2, 0]
}

Fix

Changed the default bottom from tokens.size.m (15px) to 0. This pushes the legend to the container's bottom edge, leaving the space between the grid and the legend to the axis labels. Users can still override with an explicit bottom value.

Related

Closes #21662

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