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.

tests/test_scrapers.py

@@ -1,5 +1,6 @@
 import pytest
 
+from scrapers.language_ids import LANGUAGE_IDS, get_language_id
 from scrapers.models import (
     ContestListResult,
     MetadataResult,
@@ -137,3 +138,28 @@ def test_scraper_metadata_error(run_scraper_offline, scraper, contest_id):
     assert objs
     assert objs[-1].get("success") is False
     assert objs[-1].get("error")
+
+
+EXPECTED_PLATFORMS = {"atcoder", "codeforces", "cses", "usaco", "kattis", "codechef"}
+EXPECTED_LANGUAGES = {"cpp", "python"}
+
+
+def test_language_ids_coverage():
+    assert set(LANGUAGE_IDS.keys()) == EXPECTED_PLATFORMS
+    for platform, langs in LANGUAGE_IDS.items():
+        assert set(langs.keys()) == EXPECTED_LANGUAGES, f"{platform} missing languages"
+        for lang, lid in langs.items():
+            assert isinstance(lid, str) and lid, f"{platform}/{lang} empty ID"
+
+
+@pytest.mark.parametrize("platform", EXPECTED_PLATFORMS)
+@pytest.mark.parametrize("language", EXPECTED_LANGUAGES)
+def test_get_language_id(platform, language):
+    result = get_language_id(platform, language)
+    assert result is not None, f"No ID for {platform}/{language}"
+    assert result == LANGUAGE_IDS[platform][language]
+
+
+def test_get_language_id_unknown():
+    assert get_language_id("nonexistent", "cpp") is None
+    assert get_language_id("codeforces", "rust") is None