Problem: `:CP race` on platforms without future contests (CSES, USACO)
wastes time fetching the full contest list only to discover there is
no `start_time`. The error message is also uninformative.
Solution: Add `supports_countdown` bool to `ContestListResult` (default
`True`). CSES and USACO set it to `False`. Cache the flag per-platform
so subsequent calls skip the fetch entirely. `race.lua` checks the
cached value first, then the scraper result, and shows
`"<Platform> does not support :CP race"` instead of a generic error.
Barrett Ruth
4e709c8470
