feat: race countdown support and language version selection (#346)

## Problem

Two gaps in the plugin: (1) contest pickers have no way to know whether
a contest supports countdown (race), so the UI can't surface that
affordance; (2) `:CP submit` hardcodes a single language ID per platform
with no way to choose C++ standard or override the platform ID.

## Solution

**Race countdown** (`4e709c8`): Add `supports_countdown` boolean to
`ContestListResult` and wire it through CSES/USACO scrapers, cache, race
module, and pickers.

**Language version selection** (`b90ac67`): Add `LANGUAGE_VERSIONS` and
`DEFAULT_VERSIONS` tables in `constants.lua`. Config gains `version` and
`submit_id` fields validated at setup time. `submit.lua` resolves the
effective config to a platform ID (priority: `submit_id` > `version` >
default). Helpdocs add `*cp-submit-language*` section. Tests cover
`LANGUAGE_IDS` completeness.

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