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/usaco.py

@@ -337,10 +337,16 @@ class USACOScraper(BaseScraper):
                     contests.extend(await coro)
 
             if not contests:
-                return self._contests_error("No contests found")
-            return ContestListResult(success=True, error="", contests=contests)
+                return ContestListResult(
+                    success=False, error="No contests found", supports_countdown=False
+                )
+            return ContestListResult(
+                success=True, error="", contests=contests, supports_countdown=False
+            )
         except Exception as e:
-            return self._contests_error(str(e))
+            return ContestListResult(
+                success=False, error=str(e), supports_countdown=False
+            )
 
     async def stream_tests_for_category_async(self, category_id: str) -> None:
         month_year, division = _parse_contest_id(category_id)