feat(race): add supports_countdown to ContestListResult

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.

lua/cp/scraper.lua

@@ -257,9 +257,12 @@ function M.scrape_contest_list(platform)
       ),
       { level = vim.log.levels.ERROR }
     )
-    return {}
+    return nil
   end
-  return result.data.contests
+  return {
+    contests = result.data.contests,
+    supports_countdown = result.data.supports_countdown ~= false,
+  }
 end
 
 ---@param platform string