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.

scrapers/cses.py

@@ -228,9 +228,13 @@ class CSESScraper(BaseScraper):
         cats = parse_categories(html)
         if not cats:
             return ContestListResult(
-                success=False, error=f"{self.platform_name}: No contests found"
+                success=False,
+                error=f"{self.platform_name}: No contests found",
+                supports_countdown=False,
             )
-        return ContestListResult(success=True, error="", contests=cats)
+        return ContestListResult(
+            success=True, error="", contests=cats, supports_countdown=False
+        )
 
     async def login(self, credentials: dict[str, str]) -> LoginResult:
         username = credentials.get("username", "")