[frontend] GPU comparison broken across dates with no runs

[cquil11]

When comparing GPU Configs across dates, the user is able to select a date range where there isn't data for BOTH GPU Configs on each date. This should not be allowed.

This probably needs to be computed as a separate JSON or something. For instance, we need to pre-compute all of the dates that B200 vLLM has new data, and only allow these dates to be compared.

In the case that multiple GPU configs are selected, we should take the set intersection of these computed values to compute available dates.