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/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")