PNG export: hide average latency in comparison mode

[camronh]

Problem

When exporting a PNG preview in comparison mode, the "average latency" metric is displayed alongside the score metrics. This adds noise — latency isn't a meaningful comparison axis in this view, and it clutters the chart with an extra bar group that doesn't represent a score.

There are two places latency currently appears in the comparison mode PNG:

1. As a bar group in the chart — A "Latency" group is hardcoded into the metric keys (_latency) in the comparison chart renderer, so it always shows up as a grouped bar alongside the actual score metrics.
2. In the footer text — The "X.XXs avg latency" line appears at the bottom of the image.

Neither of these should render when the PNG is in comparison mode.

Expected behavior

• Comparison mode PNG: No latency bar group, no latency footer text. The chart should only show score metrics.
• Normal (single-run) mode PNG: Average latency should continue to appear in the footer as it does today (and remain toggleable via the "Show average latency" checkbox in export options).

Context

The "Show average latency" checkbox in the export config panel should also be hidden or disabled when in comparison mode, since it has no effect.