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/constants.lua

@@ -74,4 +74,21 @@ M.signal_codes = {
   [143] = 'SIGTERM',
 }
 
+M.LANGUAGE_VERSIONS = {
+  atcoder = { cpp = { ['c++23'] = '6017' }, python = { python3 = '6082' } },
+  codeforces = { cpp = { ['c++17'] = '89' }, python = { python3 = '70' } },
+  cses = { cpp = { ['c++17'] = 'C++17' }, python = { python3 = 'Python3' } },
+  kattis = {
+    cpp = { ['c++17'] = 'C++', ['c++20'] = 'C++', ['c++23'] = 'C++' },
+    python = { python3 = 'Python 3' },
+  },
+  usaco = {
+    cpp = { ['c++17'] = 'cpp', ['c++20'] = 'cpp', ['c++23'] = 'cpp' },
+    python = { python3 = 'python' },
+  },
+  codechef = { cpp = { ['c++17'] = 'C++ 17' }, python = { python3 = 'Python 3' } },
+}
+
+M.DEFAULT_VERSIONS = { cpp = 'c++17', python = 'python3' }
+
 return M