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", "")

scrapers/models.py

@@ -50,6 +50,7 @@ class MetadataResult(ScrapingResult):
 
 class ContestListResult(ScrapingResult):
     contests: list[ContestSummary] = Field(default_factory=list)
+    supports_countdown: bool = True
 
     model_config = ConfigDict(extra="forbid")
 

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)