Save frequencies/GA panel to figure

[huddlej]

Description

Much like Auspice allows users to download the current view as a SVG file, we could ideally provide a way to download a SVG or similar file for each panel displayed in the interactive visualization. This functionality would support users who wish to include these visualizations in presentations or publications (e.g., me).

As part of the implementation, it would be excellent to have an HTTP endpoint to generate the SVG for a given panel, but this would be more complicated to implement and doesn't have existing parallels in the Nextstrain ecosystem.

[jameshadfield]

A single SVG DOM element (see screenshot) is trivial to save out to a SVG file, or have it available as a URL endpoint by itself. Composing all the small-multiple graphs together, along with the legend etc, is tricker. Not hard per se, so probably LLMs could really help with this task.